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Tue, 14 Aug 2018

How do you solve isosudoku?

Solving tips for this puzzle are hard to come by because it is so rare to see.

Firstly, it takes a while to get used to the fact that there are partial regions in the puzzle, so when you get stuck on most occasions it is because there is something related to these that you can use to whittle down the options.

But don't lose sight of using the usual sudoku regions too to make most of the deductions.

When it comes to the partial regions then the key is to proceed with care to stop you making mistakes. For instance with a region of length 9 in a 9x9 puzzle we are all used to seeing that if only one cell can hold a 9, then it must go there.

However in a partial region whose dimensions are less than that of the puzzle this rule cannot be applied, because it may be that number does not appear in that partial region at all!

The hardest rule to spot perhaps and sometimes the key to solving the harder isosudoku is to use the generalised case of this rule: namely, if there are n possible candidates for a region of length n, then each of those candidates must appear in exactly one cell.

What this means in practice is this: imagine we have a region containing 8 cells. The number 9 has already been eliminated from all of the 8 cells by the values in the rest of the puzzle. Now we *can* say that each number from 1 - 8 must go in each cell in the region and so if there is only one cell that can contain 8 in that partial region, this time we can place it.


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