A110049 a(n) is the least j such that (prime(n)#)*3^j + 1 is prime, where prime(n)# is the n-th primorial.

0, 0, 0, 0, 0, 2, 3, 3, 3, 3, 0, 5, 8, 4, 10, 1, 4, 2, 5, 1, 11, 7, 1, 2, 6, 6, 11, 2, 9, 10, 18, 9, 32, 3, 8, 18, 15, 3, 18, 1, 18, 4, 2, 24, 14, 32, 149, 12, 1, 67, 49, 56, 1, 12, 10, 15, 3, 7, 17, 5

Offset: 1

Author

Pierre CAMI, Sep 04 2005

Keywords

• nonn

Crossrefs

Cf. A002110.

Programs

Mathematica

a[n_] := Module[{p = Product[Prime[i], {i, 1, n}], j = 0}, While[!PrimeQ[p * 3^j + 1], j++]; j]; Array[a, 100] (* Amiram Eldar, Sep 11 2021 *)
Module[{j=0},While[!PrimeQ[# 3^j+1],j++];j]&/@FoldList[ Times,Prime[ Range[60]]] (* Harvey P. Dale, Jan 03 2022 *)