Longest Possible Palindrome - For Real This Time!
(Published on 13. June 2020, 12:32 by stephane.bura)
This puzzle is not as pretty as the previous one but it should be a more interesting challenge... and it's 100% more accurate! ;)
Normal Sudoku rules apply.
The gray line is a palindrome so, for instance, the digit in R3C4 is the same as the one in R8C6.
When the palindrome exits the grid, it continues on the opposite side. So the path it takes is R9C5-R1C6-R9C6-R1C7-R9C7.
Each dotted rectangle contains digits that add up to 15.
Penpa-Edit link for this puzzle to play it online.
Solution code: Row 5, column 5
Comments
on 28. September 2025, 14:25 by rich_27
In contrast, this one took me a lot less time to transpose and a lot longer to solve!
One interesting idea that occurred to me is palindromes that wrap around the edges of the board like this, but when they wrap there is an unknown number of cells in between with the constraint that both cells with line segments that lead off the grid (and only those) see all the cells in between. Feel free to make use of the idea in a future puzzle if you'd like, I wonder if you could get a palindrome that covers the entire grid that way ;)
Here's a sudokupad link: https://sudokupad.app/5t8atbp5j3
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Oh, interesting. There isn't a lot of variations around palindromes.
Maybe some additional hints about the number of missing digits could be needed but I like the idea :)
Thanks!
on 13. June 2020, 15:49 by stephane.bura
@0123coolkid Thanks :) Yes, because there is an odd number of copies of each digit. So at most you can have a palindrome of 9 x 8 + 1 cells, where the extra cell is at the center.
on 13. June 2020, 15:44 by 0123coolkid
I thought this was going to be super annoying at first, but once I was knee deep in the puzzle it was pretty great. Cool puzzle! What's the largest possible palindromic sequence in a sudoku? Is it in fact 73?
on 13. June 2020, 15:41 by stephane.bura
Fixed spelling.
