Border Square Difference
(Published on 20. September 2026, 10:00 by Phistomefel)
- Normal Arrow Sudoku rules apply.
- A border square is a 2x2 square that crosses at least one box border. For all border squares, the sums of the two diagonals differ by a multiple of 4 (0 is also a valid multiple of 4).
The puzzle is also available online via the CTC-App. Have fun solving!
Solution code: Last row
Comments
on 23. September 2026, 20:22 by DaniK
Lovely break in! I really enjoyed the puzzle
on 23. September 2026, 19:15 by emoney1374
Great puzzle! Got so close a couple times but had to rethink some conclusions to really crack the puzzle and break in. Wonderful construction
on 23. September 2026, 03:54 by goldpuffle
Incredible break in. The implication of the ruleset is surprisingly strong!
on 21. September 2026, 23:19 by antiknight
Amazing puzzle! Lovely break-in idea and solve path.
on 21. September 2026, 23:04 by amirs
What a beautifully constructed puzzle! The break-in is indeed crazy :)
on 21. September 2026, 21:19 by Fool on Hill
Beautifully done!
Saw what was going on. Solved in a 3* time, but definitely not a 3* puzzle. Solved very smoothly when I got into it. Very easy to stare at it for a long time, I think, until the idea strikes.
on 21. September 2026, 19:28 by Phistomefel
It seems I misjudged the difficulty a bit - sorry for that. Many thanks to everyone who tried to solve the puzzle! And I was delighted to read how many of you enjoyed the break-in. Thank you very much!
on 21. September 2026, 14:53 by Franjo
Wow! Ich habe lange auf dem Schlauch gestanden. Was für eine irre Zahlenspielerei! Als ich endlich kapiert hatte, was hier abgeht, war der Rest pures Vergnügen. Vielen Dank für dieses unfassbar schöne Rätsel.
on 21. September 2026, 12:29 by Saltensity
One of those puzzles where Simon would say "Take a bow!" at the end. Understanding the implications is not easy but crucial to the puzzle, so solving it was a real pleasure. Thank you and do take a bow, Phistomefel! :D
on 21. September 2026, 11:21 by Prince Myshkin
I have honestly no idea how to break into this one... Looks like I'll have to wait for the CTC video!
on 21. September 2026, 08:17 by palletpusher
Remarkably brilliant. The consequence of the constraint is so unintuitive and a joy to uncover. Not much calculation is actually required. Thanks for sharing this, Phistomefel!
on 21. September 2026, 06:36 by mihel111
So fascinating. I missed those.
on 20. September 2026, 19:38 by Philipp Huber
Das hat richtig Spaß gemacht! Es ist einfach faszinierend, wie so eine innovative Regel mit wenig Hinweisen einen schönen Lösungspfad liefert.
Vielen Dank für‘s Konstruieren!
on 20. September 2026, 18:12 by Snookerfan
Fabulous puzzle, especially the break-in! It was not clear what to look for, but after snooping around in the math behind the puzzle, I found it and it blew my mind. Very clever stuff as expected, thanks!
on 20. September 2026, 11:14 by zeniko
What a satisfying break-in! Thanks for sharing.
