Nice job, later you check the 7x7 exercise https://www.codewars.com/kata/5917a2205ffc30ec3a0000a8/

There is so much more to optimize! :)

Done after further taking those 30 seconds into 0.01seconds :D
Great kata!

PS: The comments regarding "medved test" made me concerned that there were some extremly hard tests which would break/slow my solution. There was no such test in Go but Very Hard Problems were much harder to solve than "medved tests" found in other languages.

Optimize, for one input, from 30 seconds into 0.5 seconds... still not good enough :D

Fixes It(desc, closure()) closure() variable capture which made the can solve 6x6 puzzle tests repeat the same test three or four times.

Does anyone have a simple explanation of the solution?

It's clear the exponent has to be calculated using a module multiple of four but I don't understand the maths how this is achieved.

Go translation fix

The random tests in Go are not random due to missing random seed initialization.
Some code clean up, fixed Go dot imports of standard library packages which is not standard and reformated the code with gofmt/vscode.

Fixed random tests not being random, rand.Seed() was missing.
Cleaned up the import . "math" for the standard library which is not standard.
Reformated code in VSCode which use gofmt to use the standard formating.

csharp translation improved to use standard library BigInteger class

I have brute forced my wait into a solution by doing performance analysis and thinking of clever ways to reduce the calculations which you have also done.
(Side note, when doing performance analysis/debugging I noticed that I had bug on my optimizations which you could also double check)
Still, there are generic ways to improve performance of any algorithm which you could try.

I don't know but I think you commented on the wrong exercise. This is the third part exercise and you probably want to work on the first one https://www.codewars.com/kata/54b724efac3d5402db00065e
Your solution might work there and otherwise I suggest add logs message to troubleshoot.

I think it's available now, enjoy it

The most elegant solution I have seen

Why did you use math/big here?