σ 0 ( n ) ≥ 8 ∑ n 2 1 = A
Find ⌊ 1 0 0 0 0 A ⌋ .
Bonus: Find the exact value
You may use the following approximations
π 2 ≈ 9 . 8 6 9 6 0 4 4
P ( 2 ) ≈ 0 . 4 5 2 2 4 7 4
P ( 4 ) ≈ 0 . 0 7 6 9 9 3 1
P ( 6 ) ≈ 0 . 0 1 7 0 7 0 0
P ( 8 ) ≈ 0 . 0 0 4 0 6 1 4
P ( 1 0 ) ≈ 0 . 0 0 0 9 9 3 6
P ( 1 2 ) ≈ 0 . 0 0 0 2 4 6 0
Notation:
σ 0 ( n ) is the divisor function, it counts the number of positive factors of a number.
P ( x ) is the prime zeta function or P ( n ) = p p r i m e ∑ p n 1
⌊ x ⌋ is the floor function.
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excellently done
The fancy part is that you don't even need to know what P ( 6 ) is.
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The numbers n with σ 0 ( n ) ≤ 7 are either 1 , or else prime powers p j for 1 ≤ j ≤ 6 , or else either of the form p q or of the form p 2 q for distinct primes p , q . Thus ζ ( 2 ) − A = = σ 0 ( n ) ≤ 7 ∑ n 2 1 = 1 + j = 1 ∑ 6 p ∑ p 2 j 1 + p = q ∑ p 2 q 2 1 + p = q ∑ p 4 q 2 1 1 + j = 1 ∑ 6 P ( 2 j ) + 2 1 [ P ( 2 ) 2 − P ( 4 ) ] + [ P ( 2 ) P ( 4 ) − P ( 6 ) ] making A equal to 0 . 0 1 1 8 0 5 2 . . . , and the answer 1 1 8 .