Also, don't forget to check for input validity.

And what does this mean? Too few elements provided? Too many? Non-exact amount? Anything else?

Needs random tests

Integer factorization is a duplicate to many existing katas.

The amount and details of validation required is ridiculous. It's even more tedious than the task itself.

Also, the specification for subMatrix is seriously lacking.

Errr...

• for that kind of difficulty, empty (effective) sample tests are just a big joke... (except if you really don't want any submission to your kata!)
• some typos in the description
• you should provide the arguments explicitly in the description of the methods
• subMatrix: The method name does not match at all what I can understand from the related description: discard...!??? Are you sure? Should the name rather be reduce, shrink or something like that? Or maybe just don't use "discard" (btw, that verb makes believe you expect mutation). On second thoughts, I believe I understand what you want: it is the same as getting/using a submatrix to calculate the determinant, that's it? You should reword the description of that method, then. Currently it's a bit misleading.

Another good Kata! Thanks.

My brain nearly overheated trying to get the stupid i, j, k correct for the Matrix multiplication, but I eventually got there.

IMO there are a few more test cases that need to be added (pls see my comments for your solution).

Cheers.