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Number theory Difficulty 4.9 AIME Prove it Ukraine

The number n=112011201111n = 11^{2011} \cdot 2011^{11} is given. How many divisors that are less than nn and are not divisors of nn, does n2n^2 have?

Solution

The number of divisors of n2=114022201122n^2 = 11^{4022} \cdot 2011^{22} is:
N=(4022+1)(22+1)=92529. N = (4022 + 1)(22 + 1) = 92529.
We can pair the divisors of n2n^2 in a way dd and n2d\frac{n^2}{d} where one divisor is greater than nn, another one is less. The number nn does have a pair, therefore the number of divisors that are less than nn is 9252912=46264\frac{92529-1}{2} = 46264.

The number N1N_1 of divisors of nn that are less than nn is:
N1=(2011+1)(11+1)1=24143. N_1 = (2011+1)(11+1)-1 = 24143.
So the number we are looking for is 4626424143=2212146264 - 24143 = 22121.

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