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ROSECODE 443

Central binomial coefficients

liuguangxi · Math ·

The binomial coefficients can be arranged in triangular form like this:

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
......

It is known as Pascal's triangle.

The middle coefficients of the even-numbered rows (row numbering starts from zero) form a sequence an(n0): 1, 2, 6, 20, 70, 252, 924, 3432, 12870, 48620, 184756, .... It has been proven that no number an with n>4 is squarefree.

Let f(n) be n divided by largest squarefree divisor of n. For example, 24=233 and its largest squarefree divisor is 6, so f(24)=24/6=4.

Let S(N) be the sum of all f(an) for n from 1 to N. You are given S(10)=1+1+2+1+6+2+4+3+2+2=24, S(100)=429083 and S(103)mod1000000007=258384246.

Find S(106)mod1000000007.