OFFSET
1,4
COMMENTS
Primes certified using PFGW from Primeform group.
EXAMPLE
(10^1)*(2^5-1) + 1 = 10*31 + 1 = 311 is prime, 2^5-1 = Mersenne-prime(3) so a(3) = 1.
MATHEMATICA
f[n_] := Module[{k = 1}, While[! PrimeQ[10^k*n + 1], k++]; k]; f /@ (2^MersennePrimeExponent[Range[15]]-1) (* Amiram Eldar, Jul 18 2021 *)
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Pierre CAMI, Feb 01 2005
EXTENSIONS
a(23)-a(28) from Amiram Eldar, Jul 18 2021
STATUS
approved