A306878 Number of 0 < k < n such that n-k and n+k are both nonprimes.

0, 0, 0, 0, 2, 1, 2, 3, 3, 3, 5, 5, 6, 6, 7, 6, 9, 10, 8, 10, 11, 10, 12, 13, 13, 13, 15, 14, 16, 18, 15, 18, 20, 16, 20, 21, 20, 21, 24, 21, 23, 26, 24, 24, 29, 25, 27, 30, 26, 30, 32, 29, 31, 33, 31, 33, 36, 33, 34, 41, 34, 36, 42, 35, 40, 42, 40, 40, 43, 42, 44, 48, 44, 44

Offset: 1

Author

Gionata Neri, Mar 14 2019

Keywords

• nonn

Crossrefs

Cf. A061357, A291564.

Programs

PARI

a(n)=my(s); for(c=1, n-1, s+=(bigomega(n-c)-1)*(bigomega(n+c)-1)!=0); s

Formula

a(n) = n - 1 - A061357(n) - A291564(n).