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

1,5

Links

Table of n, a(n) for n=1..74.

Formula

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

Prog

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

Crossrefs

Cf. A061357, A291564.

Sequence in context: A362559 A097242 A183202 * A161308 A161242 A161027

Adjacent sequences: A306875 A306876 A306877 * A306879 A306880 A306881

Keyword

nonn

Author

Gionata Neri, Mar 14 2019

Status

approved