Approved

I've modified it accordingly

I suggest to add the constant 99_999_989 to the initial solution:

function solution.surjections(n,k)
    return 0 % 99999989
end

Also, MOD should be local in refsol.

Fixed in this fork

Lua does not support arbitrary large integers natively. Expected results for sufficiently large n and k are computed modulo 2^64 (for example, n = 22, k = 16; refsol = -5164981666411839488). It should be clearly specified. A better approach is to return results modulo some large prime number.

Appreciate all of the translations, dear authors! I didn't want to flood the homepage with my "Thanks! Approved." comments ;)

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yeah Ill fix it in the future when i have the time, thanks for pointing out the problems.