Samsam's blog

A problem with mod
By Samsam, 11 years ago, In English

I have to get the sum of these terms (2 + 4 + 8 + 16 + ...... + 2^n) % (10^9 + 9). I know that the sum without the mod is given by the formula : 2*(1-2^n)/(1-2) = -2*(1-2^n) but I don't know how to get the sum mod 10^9 + 9. So please could you help me ?

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11 years ago, # |
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(a+b) mod c == ((a mod c) + b) mod c. a * b mod c == ((a mod c) * b) mod c. now you can do it with fast exponentiation

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11 years ago, # |
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Try the Modular Exponentiation Algorithm

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