Monday, 30 December 2013

Friday Puzzles #240

Last week's puzzle seems to have got lost in the Christmas Maelstrom.  Nothing too tricky this time round though.  Enjoy!
    #277 Heyawake – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 20 December 2013

Friday Puzzles #239

So most of my puzzles usually have an emphasis on the visuals, which then hopefully translates itself into an interesting solve.  The second part of the process usually requires a bit of polishing, which is something this puzzle could probably have benefited from a little more.  The first steps might be a little fiddly but you'll soon see why this is easy.  Oh well.

Numbers placed in circles must be the sum of the numbers (which can repeat) along the corresponding arrows.  Enjoy!
    #276 Arrow Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 13 December 2013

Friday Puzzles #238

In case you didn't see, I posted an easy masyu for last week's puzzle.  This week, something much harder.  As always with this variant, numbers placed in cells related by a chess knight's move must be different.  Enjoy!
    275 Anti-Knight Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

Friday, 6 December 2013

Friday Puzzles #237

I haven't been feeling very well recently and could well be in need of a break.  Hopefully I'll edit something in a bit later.  Sorry.


It's been all too long since I posted one of these.  Enjoy!
    #274 Masyu – rated easy
All puzzles © Tom Collyer 2009-13.

Sunday, 1 December 2013

Participation at the WSC

This is a post in a series of blogs where I'd like to engage the Sudoku community in various discussions relating to the World Sudoku Championship.

The first two posts I've made have general been exaggerated versions of opinions I'm curious about, and wondering whether I have really considered all the relevant points of view.  Certainly not, I'm pleased to say given the excellent ensuing decisions we've had.
This time I'm going to jump off the fence and have a bit of a rant about something that is really beginning to annoy me.  Admittedly the examples I am about to raise have more to do with the World Puzzle Championship (WPC), but the WSC operates under exactly the same rules.

For many years the set up at the WPC was that a country could send at most 4 people to the competition to compete.  If 4 members were sent, they formed an official team for the team rankings, and if not, there was an opportunity to mingle with other short-staffed countries to form unofficial UN teams which didn't appear in the official rankings..  Everyone who participated individually was ranked!

At the WSC the situation was a slightly different, to reflect the popularity of Sudoku and the WSC.  Teams used to be of size 3, and a maximum of 6 people per country could be sent to compete.  Again, where the numbers didn't add up, unofficial UN teams could be formed.  Again, everyone who turned up ended up with an official ranking.

The one complication came with countries who had both an A and a B team.  What if the B team's ranking in the team competition was higher than the A team's ranking?  Well, then the B team's ranking was used to display the final result for that country.  This happened to the UK, for example, in 2009.  A completely natural and sensible compromise, you'd have thought.

In 2011 things changed when the WPC and WSC merged and players from one championship were able to have a go at the other.  A new set of rules was brought in, and in my opinion this was a change for the worse.  Given the status quo is not entirely clear, let me spell it out to you.
  1. WSC and WPC teams are defined as 4 players and possibly 1 non-playing captain.
  2. Where a country sends 1-3 players, they will appear in the official individual rankings, but the country will not appear in the official team rankings.
  3. A country can send at most an A and a B team.
  4. Only the A team will be considered in the official team rankings.
  5. The formation of UN teams, including from guests, is at the host's discretion.
  6. B team members aren't included in the official individual ranking.
  7. Guests aren't included in the official individual ranking, but may appear in the full individual rankings if they are in a UN team (since individual scores contribute to team scores).
Perhaps some of you, like me, are a little bemused by all this talk of unofficial and official rankings.  Aren't all these rules overcomplicating things a little bit?  Should alarm bells be ringing?

The best 10 solvers at the 2011 WPC were, objectively:
Ulrich Voigt5075 points
Palmer Mebane4769
Thomas Snyder4546
Hideaki Jo4280
Bram de Laat4189
Peter Hudak4174
Michael Ley4062
Nikola Zivanovic3974
Roland Voigt3967
Wei-Hwa Huang3896
Is it not very strange then, that Michael Ley was not allowed to be one of the 10 finalists, and instead my compatriot Neil Zussman was - even though Neil's points total of 3864 was nearly 200 down on Michael's?

The reason was that Michael Ley was on the German B team.  And so his results were null and void.  They didn't count.  He may as well have not bothered.  Of course this situation was known to everyone beforehand, and Michael will be the first to admit that participation is equally as important at the result.  Even so, I'd argue it makes a mockery of the notion of a world championships if some of the best solvers in the world are not getting a fair chance at the title.

We were fortunate enough not to have a situation like this in 2012, but let's examine the situation in 2013.  Objectively, the best 10 solvers at the WPC were:
Palmer Mebane6060 points
Ulrich Voigt5169
Hideaki Jo5127
Thomas Snyder5107
Bram de Laat4893
Ken Endo4797
Ko Okamoto4776
Qiu Yanzhe4553
Peter Hudak4506
Kota Morinishi4301
And who were the 10 finalists?  Well, no sign of Ken Endo, who was on the Japanese B team, and so his results were null and void.  No sign either of Ko Okamoto, who wasn't even on the Japanese B team, but was instead competing as a guest, and a member of a UN team.  He had an individual score in the results because individual results are counted towards the final team score.  But to add an extra element of farce, he wasn't even give a ranking!  Qiu Yanzhe was listed as 6th on the official list, and 7th on the "all" (hahahaha) list - despite the fact his was objectively the 8th best score.  Count them!

Instead we had the 11th and 12th best solvers, Sebastien Matschke (4251 - nearly 550 down on Ken) and Will Blatt (4240 - over 500 down on Ko) making up the final.  Both are great solvers, but 500 points is the equivalent of an entire round.  Go figure.

I think everyone can agree that these situations are immensely undesirable.  I'd go so far as to say they were embarrassing.  So what is there to be done?

Well, some might say nothing is broken at all.  In 2011 Germany were at fault for not selecting the best A team, and ditto Japan in 2013.  Everyone knew the rules.

But what if I were to say the rules were stupid.  What if I were to say I think things would be better another way.  What if everyone who competed in the WSC was actually given a ranking!?

Why not operate like the WSC used to?  That is to say:
  1. WSC and WPC teams are defined as 4 players and possibly 1 non-playing captain.
  2. Where a country sends 1-3 players, they will appear in the individual ranking, but not the team ranking.
  3. A country can send at most an A and a B team.
  4. The highest scoring team (A or B) will represent a country in the team rankings.
  5. The formation of UN teams, including from guests, is at the host's discretion.
  6. B team members are included in the individual ranking.
  7. Guests are not permitted to compete in individual rounds, but can help make up the numbers for a UN team at the host's discretion.
Given that proposals to stop spitting in the face of anyone competing as a B team member and actually give them a ranking have been continually voted down at WPF general assemblies, perhaps my opinion is a minority opinion.

But I also believe that if this really is the way the WPF thinks, then they should stay consistent to their principles and explicitly limit participation to 4 people per country, and end the farces that we've seen in 2011 and 2013.

I believe my proposal is better.  And I'd love to know if people can tell me where I am wrong.

Friday, 29 November 2013

Friday Puzzles #236

I'm quite fond of this irregular layout.  Hopefully there'll be some more WSC discussions too.  I'm sure you'll have noticed that clumsy linkbar I've added there at the top.  Enjoy!
    #273 Irregular Sudoku – rated medium

All puzzles © Tom Collyer 2009-13.

Friday, 22 November 2013

Friday Puzzles #235

    #272 Consecutive Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 15 November 2013

Friday Puzzles #234

So you may have noticed that a lot of my Sudoku feature 24 givens, which is a good symmetric number lending itself well to aesthetically pleasing designs, whilst providing plenty of flexibility to get a unique solution out.  You have to be a little more ingenious as you start dropping the number of givens, so I thought this week might be a nice opportunity to try 22.  Although you perhaps have to be a little more wary of the patterns you pick, it seems 22 isn't so much of a problem, and this week's puzzle isn't so hard either.  And perhaps it tells you something about my standards that I've chosen to publish this puzzle here.  Enjoy!
    #271 Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 8 November 2013

Friday Puzzles #233

Can't say I've been blessed with too much inspiration this week, Dearest Reader.  I've yet to make my mind up whether this variant is any good.  I suspect it might be, I've just failed to make a puzzle that really uses the constraint in a neat and imaginative way.

Anyhow, if you've forgotten the rules, consecutive numbers on the lines must differ by at most 1.  The more mathematically minded of you won't need reminding this includes 0.  Enjoy!
    270 Chinese Whispers Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Wednesday, 6 November 2013

Endurance at the WSC

This is a post in a series of blogs where I'd like to engage the Sudoku community in various discussions relating to the World Sudoku Championship.

So I was pleasantly surprised by the wealth of wisdom and good discussion that followed on from my first post, and I hope that continues.  For this post I'd like to pick up on one comment from that discussion that opens up a new discussion:
my 2 cents to begin with, dont have too many rounds. it becomes a test of endurance rather than a test of your solving skills.
In my eyes endurance comes down to two different issues.  Firstly, there is the number of rounds, and secondly there is the relative length of these rounds.

To begin to address the first issue, it is helpful to examine the typical schedule of a WSC.  This has varied from year to year, but generally the entire competition, including individual rounds, team rounds and play-offs has fit into two days of competition.  A day is typically split up into morning, early afternoon and late afternoon sessions.  I've done a little (hasty - please point out any mistakes) research and summarised the information in a table.
2013Beijing5h 40m(7 rounds)1h 25m(3 rounds)1h 45m
2012Kraljevica4h 30m(7 rounds)0h 55m(2 rounds)3h 00m
2011Eger6h 40m(10 rounds)2h 20m(2 rounds)1h 00m
2010Philadelphia6h 05m(10 rounds)3h 10m(5 rounds)1h 00m
2009Zilina4h 10m(5 rounds)3h 45m(4 rounds)1h 15m
2008Goa5h 00m(7 rounds)1h 30m(2 rounds)?
2007Prague5h 30m(6 rounds)2h 00m(2 rounds)2h 00m
2006Lucca4h 15m(8 rounds)--2h 00m
In terms of the first issue, we can see that a WSC is decided over roughly 5-6 hours of competition, with not too much variance.

This brings me on to the second issue, length of rounds.  In my research there were basically 3 types of round.  The first are shorter, 10-30 minute rounds, which are typically sprints or one-off novelty.  The second is the more bread-and-butter style rounds which are typically 35-60 minutes.  The third seems to be very much the exception, the longer 90+ minute rounds.

As far as I can tell, there have only ever been 3 such rounds: the 120 minute round from Zilina, and the two 88 minute rounds in Beijing.  In contrast, I found it remarkable that no round was longer than 45 minutes in Philadelphia - which I have long regarded as the gold standard for a WSC.

My first post has already discussed the potentially skewing effects longer rounds can have, given they inevitably feature many harder variants; instead I'd like to look at things from an endurance point of view.

The first thing to say is that every now and again most solver will have a bad round.  The days are long and intense at a WSC, you are perhaps battling the effects of jet lag and so it seems inevitable that your concentration will lapse.  If this happens half way through a particular round, then the longer the round is, the more you will be punished.

A slightly different angle, which every solver is familiar with, is when you get halfway through solving and you find that you have made a mistake.  I know that when this happens to me it can often cause me to lose focus and confidence and affect the rest of my round - particularly when the puzzle is worth lots of points and I don't want my time to have been completely wasted.  Again, the longer the round is, the more you can end up punished.

The argument then goes that if you have more and shorter rounds, there is more chance to reset your mind and recover during the breaks, and approach the round after a bad one in a much better frame of mind.  One bad round doesn't have to make or break your championship.

As a counter-point, I'd also like to remark that most online competitions are typical 120 minutes long, and might be argued to be bigger tests of endurance than any single WSC round.  But regardless of the length of the competition, we tend to see the same old names at the top the majority of the time.

I could go on for longer, particularly with regards to WPC influence where rounds are often longer, but I think now is the appropriate moment to let my audience make up their own minds, and offer their own perspectives and insights into the issue of round length.  I look forward to your comments!

Friday, 1 November 2013

Friday Puzzles #230

Chronological order is overrated.  For the week before last's puzzle, I thought I'd introduce all my dearest readers to the wonderful puzzle that is curve data.  I'll try and write up a rigorous version of the rules soon, however part of the beauty of this puzzle is just how intuitive it is, so I'm sure you'll all soon work out what is going on.

Re uniqueness, this seems to be a bit like numberlink for me, so perhaps bear with me on some future puzzles.  I'm pretty confident these are all good though!

The first puzzle is a somewhat narcissitic-yet-trivial example - although hopefully it clarifies exactly what you are supposed to do with loops.
    #268 Curve Data – rated (stupidly) easy
Maybe this next one isn't quite a medium, but seeing as it's the first one for the blog I'm going to relax my strict regime just this once!

EDIT: A previous version had rather more solutions than I thought.  This one I think is unique!
2nd EDIT: It isn't!  Back to the drawing board... :)
3rd EDIT: I got someone to test solve this first :p
    #269 Curve Data – rated medium
All puzzles © Tom Collyer 2009-13.

Friday Puzzles #232

I'm going to keep this brief, Dearest reader, so I don't take too much attention away from my previous post.  Only even numbers are allowed in shaded squares.  Enjoy!
    #267 Even Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Points scoring at the WSC

This is the first post in a series of blogs where I'd like to engage the Sudoku community in various discussions relating to the World Sudoku Championship

For this first post, I'd like to talk about points scoring at the WSC.

The traditional model of the WSC goes like this.  The organisers will fashion together a set of puzzles of various types, and then group them into a number of rounds.  The puzzles will be test solved by a number of testers, and their times will be aggregated, and each puzzle will consistently be assigned a number of points according to how they tested.  For example, if we have decided each championship minute should be worth 10 points, a 2 minute puzzle will be graded as being worth 20 points.

I think there are a few things worth discussing here, but the first I'd like to concentrate on is that there are Sudoku puzzles, and there are Sudoku puzzles.  For the former, read things that are generally well known and recognised as Sudoku, your classics, diagonals, extra regions, odd/evens, irregulars and killer.  For the latter, you are almost encroaching onto WPC territory with things like Greater Than, Kropki and Skyscrapers - all of which exist as standalone Latin square puzzles without Sudoku's trademark Third Constraint.

At every WSC round there is inevitably (at least one) long round, typically 45 minutes to an hour, or even longer, full of challenging Sudoku variants (see also the Daily League project) that if you were to show them to the general public, you could be sure of a reaction of bemusement and bafflement.  Due to the length and difficulty of these rounds, they are usually the rounds that score the most points and decide who finishes where in the classification.

As such, the winner of the WSC tends to be the best all-round solver, combining a mix of WPC skills and raw Sudoku solving speed.

I think the balance used to be weighted much more to WPC skills than it was.  For example, in 2006 Rachel Roth-Huber bested David McNeill (and my good self!) at the Times Su Doku Championship.  But the following spring at the WSC in Prague, David - a WPC veteran - finished 4th, compared to Rachel who finished way back in 66th (for reference, I was 45th).  This is not to say David does not possess exceptional raw Sudoku solving speed (he certainly does), but I would certainly have forgiven you for expecting the gap between the two at the WSC to be a little closer.

These days, the WSC play-offs form a large (although not perfect) overlap between the best all round solvers and the quickest classic solvers, which I think is probably testament to the fact that Sudoku solvers have generally improved their puzzling skills.

However I'd like to finish by asking my audience to question this paradigm.  What if it were decided to place (for the sake of argument) a 0.8 multiplier on the long variants rounds at the WSC, to place more emphasis on the friendlier, more publicly recognisable puzzles?  Would this be a good idea?  And do you have any other thoughts regarding WSC scoring?

I'd love to know everyone's thoughts on this, no matter what kind of solver you are!

Friday, 25 October 2013

Friday Puzzles #231

Greetings to all my dearest readers, and firstly apologies tor missing the past 3 weeks.

On the 4th of October I had intended to put out a Twodoku, but late in the design process found a problem with the puzzle and had no time to fix it before I had to travel to Birmingham for the weekend.  I'll see what I can do with that to publish it soon!

On the 11th October I was travelling out to China to participate in the 22nd World Puzzle Championship and 8th World Sudoku Championship.

On the 18th October, a wonderful week was coming to end, and I was much too busy to be able to put out a puzzle.  Or find away to access the blogger service in China!

Anyhow, you can certainly expect some kind of championship report in the next week or so, but for now, let me crack on with Daily League business.  This week, blackout sudoku, which shouldn't prove to be as challenging or fiddly as this type potentially can be.  In fact it's probably safe to say that you won't be seeing too many more challenging Sudoku puzzles from me until sometime next summer.  Enjoy!
    #266 Blackout Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

P.S. If you've ever wondered what a bunch of Sudoku authors, from Monday to Sunday, looks like, wonder no more!

Friday, 27 September 2013

Friday Puzzles #228

Most of my Killer Sudoku end up having quite a pronounced extra-regions feel to them, which I think is something that really sets them apart from the crowd.  However, this week I wanted something more along the lines of a decent (non-computer generated) Kakuro.  The implied similarities are more subtle and less trivial than the fact both Killer and Kakuro involve adding up, as I'm sure all my Dearest Readers already know, even if they have happened to have been historically unmoved by Kakuro.  Enjoy!
    265 Killer Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 20 September 2013

Friday Puzzles #227

Thermo sudoku this week.  Numbers placed must increase from the bulbs.

Karel has also asked me to mention that there is going to be another Sudoku Cup this weekend - more details here:  And whilst you're there, you'll doubtlessly want to play Daily League puzzles online.

EDIT: please note an earlier version of the puzzle had the wrong length of thermometers in Row 1.  Sorry!
    #264 Thermo Sudoku – rated medium

All puzzles © Tom Collyer 2009-13.

Friday, 13 September 2013

Friday Puzzles #226

Anything Bastien can do...

    #263 Diagonal Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 6 September 2013

Friday Puzzles #225

Outside Sudoku this week - the clues outside the grid are placed somewhere within the first three cells in the corresponding direction.  There is a backlog of Daily League PDF's that hopefully I'll be able to get to tomorrow.  Nothing too hard here, although you'll want to watch your step at a couple of points.  Enjoy!
    #262 Outside Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 30 August 2013

Friday Puzzles #224

I think, Dearest Reader, you'll learn something new when solving this puzzle.  Enjoy!
    #261 Irregular Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

Friday, 23 August 2013

Friday Puzzles #223

Nothing special this week, classic sudoku.  Keep an eye out for more puzzles from me over at The Art of Puzzles though!

Here are some PDF's.

Week 29
Week 30
Week 31

Week 29 and week 30 aren't done very well, but I've been trying to do them for ages and they were downright impossible.  That's life.
    #260 Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 16 August 2013

Friday Puzzles #222

I've made it to Nelson again, dearest reader, and I've got nothing prepared.  There's a huge backlog of Daily League PDF's in the works - Week 29 has me temporarily beat although 30 and 31 shouldn't be too long in appearing.  Let's see.

Anyway, more extra regions today.  I've always enjoyed playing with the geometry of Sudoku grids by getting rows and columns interacting in more interesting ways than you get in a classic puzzle, and this puzzle illustrates that philosophy nicely.  Sadly this one has lost quite a few of its teeth in trying to get a unique solution, but there's enough meat on this one to keep most of my Dearest Readers entertained for a while.  If you'll excuse that horribly mixed metaphor.  Enjoy!
    #259 Extra Regions Sudoku  – rated medium
All puzzles © Tom Collyer 2009-13.

Thursday, 15 August 2013

Friday Puzzles #221

Apparently I was too busy not getting a job in London last week to remember to post a puzzle.  Have no fear, I'll be back with a bang tomorrow!

This week, a quirky Heyawake puzzle, a type I think I haven't revisited for a while.  I was tempted to go wacky on this one to make my original design work, but I think a little perseverance and polish and general puzzling discipline is good for one's authoring abilities.  Under the old system this is certainly hard, but rules is rules and it's not too hard to see where to look to continue your solving path at the sticking points.  Enjoy!
    #258 Heyawake – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 2 August 2013

Friday Puzzles #220

More daily league.  This is actually solvable (just about!) taking the 4 digits out of box 3 and box 7, but presented as is I think is a more pleasant solve.

Week 27
Week 28

Week 29
Saturday (Diagonal, Bram)
Sunday (One From Eight, Seungjae) This is very challenging!!!
PDF (on its way!)

Week 30
Monday (Pairs, Jakub H) Beware the converse rule!
Tuesday (Dominoes, Prasanna)
Wednesday (Frame, Bastien)
Thursday (Even-Anti Knight, Rishi

    #257 Non Consecutive Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 26 July 2013

Friday Puzzles #219

Even sudoku this week, which means that only even numbers are allowed in the shaded cells.  Nothing too challenging!

I hope you've been keeping up with my puzzles on the Art of Puzzles.  Thus far we've had:
And later today there's a classic Sudoku to look forward to later today, before a rather challenging Yajilin for tomorrow.

Re league business, I'm finishing off the week 27 and 28 pdf's so they should be available later today as well.
    #256 Even Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Monday, 22 July 2013

Friday Puzzles #218

Yeah so I have a better excuse this week - Friday I was graduating and I've been moving house and settling in over the weekend.  But better late than never, eh Dearest reader?

Firstly exciting news for those of you who aren't so keen on sudoku and its variations.  I am now officially a contributor over at Grandmaster Puzzles.  There'll be a puzzle from me appearing every day until Saturday.  Which is nice!

Some daily league bookkeeping.  (You have to love the triple double-letters in that word!)

Week 27
Saturday (Irregular +2, Hns Eendebak)
Sunday (Sequence, Seungjae)
PDF (coming soon)

Week 28
Monday (Rank, Christoph)
Tuesday (Windoku, Karel)
Wednesday (Pinocchio, Bastien)
Thursday (Linked Irregular, Jakub H)
Friday (Hi-Lo Inner Frame, Bram)
(mine counts as Saturday!)

Don't forget that you can play with a day's delay over at

Back to this (last? haha) week's puzzle.  I wrote this in a rush and the visual theme isn't quite developed to my satisfaction and the solve is fairly straightforward.  It sows the seeds for better things in the future.  Enjoy!
    #255 Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Tuesday, 16 July 2013

Friday Puzzles #217

Seems I was very naughty on Friday and forgot to make a blog post.  Rest assured dearest reader, it did appear on the facebook group in good time so if you've been following the daily league and haven't yet joined up, then why not?

Week 26
Saturday (Gapped Non Consecutive, Gui Yong)
Sunday (Anti-Knight, Seungjae)

Week 27
Monday (Triangle Sums, Zoltán Horváth)
Tuesday (Frame, Prasanna)
Wednesday (Non Consecutive Anti-Knight, Bastien) - this is a very powerful combination of constraints!
Thursday (Renban Groups, Jan Zvěřina)

As always, you can play with a day's delay on the servers.

This week's puzzle is a difficult diagonal sudoku.  Probably too hard for competition.  Enjoy!
    #254 Diagonal Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

Friday, 5 July 2013

Friday Puzzles #216

Daily League Update!

Week 25
Saturday (Ace, Bram)
Sunday (Missing Arrow, Seungjae)

Week 26
Monday (1011, Christof) - and it's definitely worth noting this variant is combinatorially equivalent to consecutive...
Tuesday (Point-to-Next or Search 9, Prasanna)
Wednesday (Tile, Bastien)
Thursday (Classic, Jakub H)

Look, Dearest reader, I've finally got around to doing a proper update!  My offering this week is an irregular puzzle that kind of challenges my rating system a little because I know many people find these types to be quite tricky, often because the solving path is rather narrow.  The same is true of this puzzle, but I think this would be fair game as a competition puzzle and so I cannot give it the label hard.  Even though this is definitely harder than last week's medium.  Apparently I can't win.  Enjoy!
    #253 Irregular Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 28 June 2013

Friday Puzzles #215

Christof S posted a beautiful Klein bottle sudoku earlier this week.  This led me to revisit my own dabbling with sudoku on various square identification spaces.  My own 9x9 version of the Klein bottle was somewhat flawed as although my rows were all of length 9, I had 4 rows of length 18 and one of length 9.  By setting things up as a 10x5 grid Christof has come across a very elegant solution.

Unfortunately I have not even begun to scratch the surface transferring over the images that used to exist on the Warwick servers over to blogger so you (currently) can't see my own Klein bottle puzzle, and nor can you see my interpretation of the real projective plane.  That wasn't very good either in retrospect, bearing in mind there were now four rows and four columns of length 18, and just the one row and column of length 9.

I've decided to rescue this Christof-style by having a much smaller grid.  Correctly interpreted, this grid has three rows, three columns and three regions of area 12.  Rather than ask you to enter in the numbers from 1-12, I'm asking you to enter them in modulo 6.  This ambiguity means I don't need a minimum of 11 clues.  Here I get away with 10, perhaps with more cunningly shaped regions you could have fewer.

That's probably as confusing to most of my Dearest Readers as the notion that the real projective plane is simply the quotient of a regular 2-sphere by the antipodal map, so you can also view the puzzle this way: a standard 6x6 Latin square with 3 regions of area 12 in which each number from 1-6 appears exactly twice.  Given the size, this isn't particularly hard, but it's also in no way as trivial as a standard 6x6 sudoku.  You'd definitely better be sure that exactly two of each number are in each region.  Enjoy!
    #252 Topological Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 21 June 2013

Friday Puzzles #214

More daily league sudoku - but perhaps you might be able to see some other puzzle types from me elsewhere sometime soon.  More on this to come, hopefully.

Don't forget you can play online with a day's delay at!

This week is a continuation of last week's idea.  I think this is a variant with lots and lots of potential.  This isn't a particular beautiful puzzle to look at, or to solve, but at least it should provide something of a challenge.  This certainly took a while to test solve last night! 

Oh and the rules: consecutive terms of the sequence to be place on the marked line should differ by at most 2.  Which means they can differ by 0, 1 or 2.  Enjoy!
    #251 Chinese Whispers Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

Friday, 14 June 2013

Friday Puzzles #213

I do keep promising a proper daily league update, but I've been a little bit busy recently.  I'm sure you don't need reminding dearest reader, but my good friend Karel has asked me to remind you all that you can play along online via his rather wonderful website.

This week consecutive-on-line sudoku.  This means that numbers placed along each marked line should form a sequence (not necessarily arithmetic!) where consecutive terms differ by 1.  I've seen this before under various other names, but I couldn't initially remember and had the name Chinese whispers sudoku mentioned to me.  However this seems more suitable where consecutive terms differ by at most (rather than exactly) n, for some sensible value of n.  More on that next week I suppose.  For now, Enjoy!
    #250 Consecutive-on-line Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 7 June 2013

Friday Puzzles #212

Keep it simple, stupid.  I'll do a daily league update some other time.  Enjoy!
    #249 Sudoku – rated easy
 All puzzles © Tom Collyer 2009-13.

Tuesday, 4 June 2013

Friday Puzzles #211

It's just as well I forgot to post this on Friday, as it turned out to be something of a disaster.  The idea here is that adjacent cells do not contain numbers whose sum is 5 or whose difference is 5.  Combinatorially speaking, this is equivalent to an XV Sudoku without any clues.  This probably explains why there were a few problems, and my thanks go out to Karel Tesař (of fame) for helping to fix this puzzle.  This is really his puzzle! 

Anyhow, better late than never.  I've already got this week's puzzle ready so no worries there.  Enjoy!
    #248 Number 5 is not Alive Sudoku rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 24 May 2013

Friday Puzzles #210

The good old non consecutive constraint provides at once both great delight and pain for the aspiring puzzle designer.  Perhaps what I originally had in mind for this puzzle will become slightly more apparent when you have solved this.  With the aid of a program, I started with my gimmick and a few randomly specified clues and I found a valid solution grid.  Whilst trying to maintain the gimmick and carefully manipulating the givens to yield a bona fide puzzle I found I either ended up with something that was either too hard (by which I mean tediously so) or else too easy.  So the gimmick went, I got a little creative and eventually Dearest Reader I arrived at something which was to my satisfaction.

In lieu of last week's puzzle I have also decided to give my difficulty ratings a little rethink.  From now on "easy" is going to be applied to puzzles which can be solved with standard and well-known steps.  These will generally take experienced solvers no more than 5 minutes to solve.  "Medium" will be applied to puzzles with perhaps a novel step or two or else a couple of bottleneck steps which definitely need to be resolved before further progress can be made with the puzzle.  There will be some variability in the times here, with solving times encroaching both below into easy territory and above into hard, reflecting the increased variance in solving inevitably introduced by novelty.  "Hard" will be largely be reserved for innovative puzzles requiring involved non-standard steps or with a solving process with several bottleneck steps.  These will generally take even experienced solvers (at least) a good 10 minutes to solve.

Please note this will mean a great deal of formerly "medium" puzzles will be downgraded to "easy,"  and ditto many "hard" puzzles will become medium.  Although there will of course be some puzzles in both categories that survive the cut!

I'll possibly retrospectively go back and edit the difficulties in the archive of previous entries at some point.  In more generality it's something I need to start doing at some point so that the image files are hosted on blogger rather than at warwick.  I might even start tagging them!
    #247 Non Consecutive Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 17 May 2013

Friday Puzzles #209

After UKSC duty, I'm back to the daily league.  I'm going to sit down and do a bit of an update of all PDF related stuff tomorrow.  In case you were wondering, we are up to Week 19 now - and scattered through the facebook group are links to the PDF's up to week 17.  Week 18 will come tomorrow as well, honest!

So I was up til about 4am tweaking this puzzle until it was just right.  In case you were wondering whether I can do hard puzzles, here's an extra regions puzzle which will push your geometric understanding of the added constraints quite far.  Let me warn you now, until you work out exactly what is going on, you aren't going to make much progress.  Puzzles like these where there are no freebies at the start, and where every given plays its own crucial role both by itself and in ensemble are very hard to come by, and I think this is probably as good a puzzle as anything I saved for the UKSC.  If not better!  And whilst I'm here blabbering on, I'm also going to take this opportunity to also point you towards Bastien's wonderful 16 given Diagonal

    #246 Extra Regions Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

Friday, 10 May 2013

Friday Puzzles #208

Sorry, I've been busy with other things today/this week.  Expect another yajilin puzzle in the not-too-distant future!

Sorry for the delay.  Easier than last week, but watch your step!
    #245 Yajilin – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 3 May 2013

Friday Puzzles #207

I'm fairly sure, Dearest Reader, that you won't have seen many yajilin puzzles like this before.  Enjoy!

EDIT: Original posted puzzle had multiple solutions; please see the comments below for the discussion.  First "correction" had a counting error!
    #244 Yajilin – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 26 April 2013

Friday Puzzles #206

So, Dearest Reader, I'm taking a break from Sudoku and the league for now for the next couple of weeks.  Week 14 PDF and Week 15 PDF.

Maybe I'll have time to explore some puzzle ideas, which maybe you'll be getting to see soon either here or elsewhere.  For this week, an old favourite.  Sometimes I enjoy the cute perturbations to the theme that make things work almost as much as pursuing the perfect twisted theme.  Enjoy!

EDIT: On a related Masyu note, I thoroughly recommend this contest running over the next few days by the fine folk at LMI.  You'll find a couple of puzzles with a little more bite than this one!
    #243 Masyu – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 19 April 2013

Friday Puzzles #205

Palindrome Sudoku this week.  I suppose, Dearest Reader, you are clever enough to know that means putting palindromic sequences along the lines in the grid.

I'll edit in all the League stuff later.

In other news, for the next few weeks I'm taking some time off from the League, so you'll also be seeing some puzzles which aren't Sudoku for the first time in a while.  Which is nice.  Enjoy!
    #242 Palindrome Sudoku – rated easy
 All puzzles © Tom Collyer 2009-13.

Friday, 12 April 2013

Friday Puzzles #204

More classic sudoku this week, although more exciting sudoku news soon.  Enjoy!

Week 12

Week 13
Saturday (Windoku, Bram)
Sunday (Prime Number, Seungjae)
PDF (coming soon)

Week 14
Monday (Diagonal-Untouch, Fred)
Tuesday (XV, Prasanna)
Wednesday (Sudokurve, Bastien)
Thursday (Non Consecutive, Rishi)
    #241 Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 5 April 2013

Friday Puzzles #203

Ok, let's do this week's League puzzle properly.  Starting with a lovely new logo:

This week an easy classic, which might possibly become a running theme for the next couple of weeks as all my more interesting puzzles are being eaten up elsewhere.  Still, everyone likes a race, no?  Here's an added pro tip, relevant to the design: it's never ever what would seem nice.  Enjoy!

Week 12
Saturday (Average, Bram)
Sunday (Little Variation Killer, Seungjae)
PDF (coming soon!)

Week 13
Monday (Knightly Non-Consecutive, Fred).  Not the killer!
Tuesday (Disjoint Groups, Prasanna)
Wednesday (Extra Region with Mirrored Cells, Bastien)
Thursday (Diagonally Non-Consecutive, Rishi)
    #240 Sudoku – rated easy
 All puzzles © Tom Collyer 2009-13.

Friday, 29 March 2013

Friday Puzzles #202

I'm very very tired.  Tens sudoku.  Adjacent cells containing numbers summing to ten are both shaded. Adjacent cells which are not both shaded must not contain numbers summing to ten.  This is pretty hard.  Enjoy!
    #239 Tens Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

Friday, 22 March 2013

Friday Puzzles #201

It's been a year or so since I posted one of these.  I seem to have something of a penchant for sudoku variants where you alter the grid slightly to enter a smaller subset of digits.  I suppose the thing with this variant in particular is that the star-battle constraint is fairly strong in a 9x9 grid, and especially so when you restrict them to 3x3 boxes.  So, maybe a bit of thought to get started, but nothing too strenuous afterwards.

The rules, dearest reader, in case you've forgotten: 1-8 in each row, column and 3x3 box, with 2 appearing twice in each.  Moreover, the cells containing 2's must not touch either adjacently, or diagonally at a point.  And yes, it'd be more natural to do 1-7 with two stars, but "two" rhymes with "su" and that's more than enough to turn me on.

I'll edit in league updates later, but I must confess it's something of a chore.  Why not help me out and follow all the league updates via the facebook group?

And seeing as I didn't post it last week, let me link you to the week 9 PDF.  I'll edit in the week 10 PDF on Monday, or something.

    #238 Twodoku – rated medium
All puzzles © Tom Collyer 2009-13.

Saturday, 16 March 2013

Friday Puzzles #200

Seems I forgot to post yesterday.  Here's a diagonal sudoku for the daily league.  Have you tried playing online?
    #237 Diagonal Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 8 March 2013

Friday Puzzles #199

This week we have a puzzle whose better known cousin involves the number 5.  I've no idea where the name comes from, I'm not sure I like it, but basically this is killer sudoku where the cages aren't labelled.  All you know is that the units digit is 8 - so to be completely clear the only possible values are 8, 18, 28 or 38.

Still, I'm not one to turn down a request, and this should hopefully prove to be a good warm up.  As with many of the slightly more obscure sudoku variants, a lot of the difficulty comes from the novelty, so beginners might find this a little harder than medium as they get to grip with logic the constraint adds.  Enjoy!

Week 8:
Saturday (Quad Second, Bram)
Sunday (Magic Summer, Seungjae)

Week 9:
Monday (Arrow, Fred)
Tuesday (Search 9, Prasanna)
Wednesday (Renban, Bastien)
Thursday (Odd-Even Chessdoku, Rishi)
    #236 A Date with 8 – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 1 March 2013

Friday Puzzles #198

You should know the drill by now.  This week another classic puzzle, which kind of defies a sensible rating.  I have good reasons to equally call this easy, medium or hard.  You'll hopefully see what I mean after you've solved this, dearest reader. Enjoy!

Week 7:
Saturday (Scattered, Bram)
Sunday (Multiplication Table, Seungjae)

Week 8:
Monday (Killer Pair, Fred)
Tuesday (Point To Next, Prasanna)
Wednesday (Greater Than Killer, Bastien)
Thursday (Non Consecutive-Anti Knight, Rishi)
    #235 Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 22 February 2013

Friday Puzzles #197

Odd Sudoku this week - that is only odd numbers can go in shaded cells.  I don't care what Fred says, I think this is pretty hard.  Constructing this was a lot like constructing a Sudoku Islands puzzles, and I imagine is how people go about trying to construct the monstrously difficult scanraid-busting classic puzzles.  Enjoy!

Week 6:
Saturday (V day, Bram)
Sunday (Counting on line, Seungjae)

Week 7:
Monday (Equal, Christoph Seeliger)
Tuesday (Between 1 and 9, Prasanna)
Wednesday (Fortress, Bastien)
Thursday (Little Killer, Rishi)

Oh, I should also mention that it's been a particular difficult week, so watch your step with these.  And if you haven't already joined the facebook group, then why not!?
    #234 Odd Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 15 February 2013

Friday Puzzles #196

More daily league.  I'll edit in further updates later, or something.  Enjoy!

Week 5:
Saturday (Battenburg, Bram)
Sunday (All Odd/Even-GT, Seungjae)

Week 6:
Monday (Blackout Sudokuro, Fred)
Tuesday (Untouch, Prasanna)
Wednesday (Odd-Toroidal, Bastien)
Thursday (Anti-Knight, Rishi)
    #233 Non-consecutive Sudoku – rated easy
All puzzles © Tom Collyer 2009-13.

Friday, 8 February 2013

Friday Puzzles #195

More daily league.

Week 4
Saturday (Little Killer, Stefan Heine)
Sunday (Bridge, Seungjae)

Week 5
Monday (Thermo, Fred)
Tuesday (Rossini, Prasanna)
Wednesday (Classic, Bastien) - highly recommended!
Thursday - (Trio, Rishi)

And finally me.  Odd-Even-Big-Small Sudoku is a beautiful Japanese variant that has a lovely blend of hidden singles and pairs in its solving flavour.  Makes solving an 8x8 puzzle vaguely bearable.

Oh, and the rules.  Where you have a clue outside the grid, the first two cells in the corresponding direction must match the relevant description.  Odd = {1,3,5,7}, Even = {2,4,6,8}, Big = {5,6,7,8} and Small = {1,2,3,4}.  Enjoy!
    #232 Odd-Even-Big-Small Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Friday, 1 February 2013

Friday Puzzles #194

More Sudoku this week, but I suppose it's all good practice for me for 2014.  I suppose I am not being too much of a tease to suggest that if you'd like to see a few more puzzles without a Sudoku flavour in the coming weeks, then I can only suggest that you join the facebook group and offer to write puzzles for the Daily League instead of me.

Here are the updates, by the way.  Quite a few of the puzzles this week are very challenging, so consider yourself warned!

Week 3:

Week 4:
Monday (Extra Regions, Fred)
Tuesday (Anti-Diagonal Sums, Prasanna)
Wednesday (Pointing Evens, Bastien) - this is a delightfully simple-yet-subtle variant, by the way.
Thursday (Argyle, Rishi)

Anyhow, one of the few benefits of being ill and retired to bed by the early evening is that I have plenty of time to put together puzzles.  On the other hand, I'm not necessarily in such a great condition to solve or grade how difficult puzzles really are.  Still, this puzzles has plenty of the sort of logic that I would judge makes an analogous classic puzzle a hard puzzle, so I shall stick with my guns on this one.  Enjoy!
    #231 Diagonal Sudoku – rated medium
All puzzles © Tom Collyer 2009-13.

Thursday, 31 January 2013

Daily League Update

So I promised to update last Friday's post with various links and stuff, but didn't quite get around to it.  SInce if I make an edit now no-one will actually notice, I've decided to make a new post, and indeed invent a new label to boot.  Here goes nothing:

Week 2:
Saturday (Distance, Bram)
Sunday (Quadruple, Seungjae)
PDF - as always, download before printing!

Week 3:
Monday (Killer, Fred)
Tuesday (Queen, Prasanna) - and yes, I am loving the juxtaposition of Killer and Queen!
Wednesday (Diagonal, Bastien)
Thursday (Irregular-Non Consecutive, Rishi)
Friday (Irregular, Me)
Saturday (No Tens, Bram)
Sunday (Mickey Mouse, Seungjae)

PDF will be ready with tomorrow's post!

P.S. If you aren't already involved then I thoroughly encourage you to engage with our facegroup group.  What have you got to lose?

Saturday, 26 January 2013

Friday Puzzles #193

When I am feeling less disorientated and delirious, I will post plenty of daily league updates, the Week 2 PDF, and a brief apology for this puzzle being hard and cool, but not nearly as hard or cool as it should have been.

For now, I'm already 10 minutes past Friday is up, although this won't trouble my dearest American readers.  If you've not already joined the facebook group, you'd have seen this puzzle a few hours ago anyway.  Enjoy!
    #230 Irregular Sudoku – rated hard
All puzzles © Tom Collyer 2009-13.

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