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PROJECT EULER · #0650

Divisors of Binomial Product

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Let B(n)=k=0n(nk), a product of binomial coefficients.
For example, B(5)=(50)×(51)×(52)×(53)×(54)×(55)=1×5×10×10×5×1=2500.

Let D(n)=d|B(n)d, the sum of the divisors of B(n).
For example, the divisors of B(5) are 1, 2, 4, 5, 10, 20, 25, 50, 100, 125, 250, 500, 625, 1250 and 2500,
so D(5) = 1 + 2 + 4 + 5 + 10 + 20 + 25 + 50 + 100 + 125 + 250 + 500 + 625 + 1250 + 2500 = 5467.

Let S(n)=k=1nD(k).
You are given S(5)=5736, S(10)=141740594713218418 and S(100) mod 1000000007=332792866.

Find S(20000) mod 1000000007.

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