A118664 a(n) is the least prime p such that n*(p#^2)-1 is prime or 0 if n > 1 is a square so no prime possible.

2, 2, 2, 0, 2, 2, 3, 2, 0, 3, 2, 2, 3, 3, 2, 0, 2, 2, 3, 2, 2, 269, 3, 3, 0, 2, 2, 7, 5, 29, 11, 2, 2, 3, 2, 0, 11, 2, 5, 3, 2, 2, 5, 3, 2, 5, 5, 2, 0, 2, 17, 3, 2, 11, 3, 2, 2, 3, 7, 2, 7, 5, 2, 0, 3, 2, 3, 2, 5, 7, 2, 3, 5, 3, 3, 5, 2, 2, 3, 3, 0, 41, 2, 3, 7, 7, 2, 3, 3, 2, 5, 2, 3, 53, 2, 2, 3, 3, 7, 0

Offset

1,1

Links

Table of n, a(n) for n=1..100.

Example

1*(2^2)-1 = 3 is prime, so a(1) = 2.
7*(2^2)-1 = 27 is composite, 7*((2*3)^2)-1 = 251 is prime, so a(7) = 3.

Mathematica

a[n_] := If[n>1 && IntegerQ[Sqrt[n]], 0, Module[{p = pr = 2}, While[!PrimeQ[n * pr^2 - 1], p = NextPrime[p]; pr *= p]; p]]; Array[a, 100] (* Amiram Eldar, Sep 11 2021 *)

Crossrefs

Sequence in context: A328995 A036476 A104994 * A223175 A322213 A118205

Adjacent sequences: A118661 A118662 A118663 * A118665 A118666 A118667

Keyword

nonn

Author

Pierre CAMI, May 19 2006

Status

approved