A305818 - OEIS

A305818

Number of proper divisors d of n such that 2d+1 is a prime.

0, 1, 1, 2, 1, 3, 1, 2, 2, 3, 1, 4, 1, 2, 3, 3, 1, 5, 1, 3, 2, 3, 1, 5, 2, 2, 3, 3, 1, 6, 1, 3, 3, 2, 2, 6, 1, 2, 2, 5, 1, 6, 1, 3, 5, 3, 1, 5, 1, 3, 2, 3, 1, 6, 3, 4, 2, 3, 1, 8, 1, 2, 4, 3, 2, 6, 1, 2, 3, 5, 1, 8, 1, 2, 4, 2, 2, 6, 1, 5, 3, 3, 1, 6, 2, 2, 3, 5, 1, 9, 1, 3, 2, 2, 2, 6, 1, 3, 5, 5, 1, 5, 1, 4, 6

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,4

COMMENTS

Number of terms of A005097 which are less than n that divide n.

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..65537

FORMULA

a(n) = Sum_{d|n, d<n} A101264(d).

a(n) = A086668(n) - A101264(n).

PROG

(PARI) A305818(n) = sumdiv(n, d, (d<n)*isprime(d+d+1));