A266715 Number of k <= floor(n/2) such that (n mod k) is prime.

0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 2, 1, 1, 3, 2, 1, 4, 3, 3, 3, 3, 3, 8, 2, 3, 6, 7, 2, 7, 4, 6, 6, 7, 4, 10, 2, 8, 9, 9, 3, 10, 6, 11, 6, 10, 4, 17, 4, 7, 10, 12, 6, 15, 5, 13, 7, 13, 9, 19, 3, 13, 12, 17, 3, 17, 7, 18, 11, 13, 7, 21, 7, 17, 12, 20, 4, 23, 6

Offset

1,11

Links

Clark Kimberling, Table of n, a(n) for n = 1..1000

Example

(11 mod k) gives 0,1,2,3,1, with primes 2,3, so a(11) = 2.

Mathematica

t[n_] := Table[Mod[n, k], {k, 1, Floor[n/2]}]
p[n_] := Select[t[n], PrimeQ[#] &]
Table[Length[p[n]], {n, 1, 200}]

Prog

(PARI) a(n) = sum(k=1, n\2, isprime(n % k)); \\ Michel Marcus, Feb 04 2016

Crossrefs

Cf. A266714, A268372.

Sequence in context: A124832 A226130 A137569 * A089177 A348445 A023996

Adjacent sequences: A266712 A266713 A266714 * A266716 A266717 A266718

Keyword

nonn,easy

Author

Clark Kimberling, Feb 03 2016

Status

approved