OFFSET
1,1
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
KEYWORD
nonn
AUTHOR
Pierre CAMI, May 19 2006
STATUS
approved