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Issue with large value
By _doomboo_, history, 10 months ago, In English

I was solving this problem with a combinatoric based approach. So I compute binomial coefficient using Fermat Little Th. and modular inverse. I have correct values for N up to 34, then my code starts giving wrong values. I belive the approach is correct but there is some issue I cant figure out related to modulo, modular inverse or powmod function. Can anybody help me understanding why my code fails? CODE

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10 months ago, # |
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Auto comment: topic has been updated by _doomboo_ (previous revision, new revision, compare).

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10 months ago, # |
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Your C(n, k) function should be:

Spoiler
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    10 months ago, # ^ |
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    it doesnt work anyway. I tried also approach with pascal triangle to compute nCk but I get the exact same issue: everything is fine for N up to 34, then it starts failing... it's so weird

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10 months ago, # |
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Nobody able to help? :(

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10 months ago, # |
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Please help

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10 months ago, # |
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I might be wrong, but the 70'th line (ll half = nCk(n, n/2, mod)/2;) might mess your calculations up, maybe that nCk returns an odd number but you divide it by 2. I suggest using *inverse(2, mod). I haven't verified the code logic so I cannot say if the approach is correct or not. Hope this helps.

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    10 months ago, # ^ |
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    thank you very much, this is the actual issue. You made my day :D.

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