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%I #10 Jul 18 2021 04:36:49
%S 1,1,1,4,2,3,10,6,28,12,45,23,36,18,114,72,652,47,39,61,3713,208,9655,
%T 965,11508,684,7085,1803
%N a(n) is the least k such that (10^k)*Mersenne-prime(n) + 1 is prime.
%C Primes certified using PFGW from Primeform group.
%e (10^1)*(2^5-1) + 1 = 10*31 + 1 = 311 is prime, 2^5-1 = Mersenne-prime(3) so a(3) = 1.
%t f[n_] := Module[{k = 1}, While[! PrimeQ[10^k*n + 1], k++]; k]; f /@ (2^MersennePrimeExponent[Range[15]]-1) (* _Amiram Eldar_, Jul 18 2021 *)
%Y Cf. A000043, A000668.
%K nonn,more
%O 1,4
%A _Pierre CAMI_, Feb 01 2005
%E a(23)-a(28) from _Amiram Eldar_, Jul 18 2021
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