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Feedback and Questions

I've received a lot of interesting comments and questions from Sudoku fans over the last few years and this page is where I try to answer them. Please feel free to drop me a note on the side of the page or try the Facebook comment box. Or you can email me directly at .

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Many thanks to all the people who have helped improve the solvers and strategies with their feedback!

Saturday 13-Feb-2021

... by: Eugene, US

I made a small donation...hopefully it helps a little with the recent hack. Hope it gets sorted out!

Andrew Stuart writes (14-Feb-2021):

Thank you! Going towards a better backup system, for sure.
Really appreciated
All the best
Andrew Stuart

Saturday 13-Feb-2021

... by: Charles R Cochems, United States

Well, seems there's a strategy that's not known by your solver. Cracking the cryptic found it. I call it odd/even. it's very usable by humans, and not any sort of bifurcation.

Divide the puzzle into odd/even columns (or rows).

In the odd columns (or rows) there must be a total of 20 evens, minus any that are already placed (4 per column).

now for each row, count the number of known evens. subtract that number from four. color that many squares that are on an odd columns. doesn't matter which ones.

now count the number of colored squares on the odd rows. if this is the same as the maximum left to place on those columns, then we can make some deductions.

Specifically if any row has all of it's cells in the odd columns marked as potential evens at this point, then you can eliminate odds from those squares. This is because placing an odd on one of them would mean there aren't enough spaces left to put all the evens.

On the rows you couldn't eliminate the odds from the odd columns, you can then eliminate the evens from the even columns on those rows, since all the evens MUST be in the odd columns. uncolor the odd cells, and color the empty even cells a different color.

This can be done with row and column swapped. Since there are less even numbers than odd, this technique usually works better with evens in the odd rows, or evens in the odd columns. smaller number, greater area.

Usually, this will get you nothing. But if you notice very few evens in the odd rows, or very few evens in the odd columns, this can give a useful result.

Andrew Stuart writes (14-Feb-2021):

I will have to look that one up on Cracking the Cryptic then, thanks for a first look

Saturday 13-Feb-2021

... by: Klaus Stärk, Switzerland

numbers 1 to 9 on the right side of sudoku?

Andrew Stuart writes (14-Feb-2021):

Some people use an alternative coordinate system row num + col num
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