A247341 Let b_k=3...3 consist of k>=1 3's. Then a(n) is the smallest k such that the concatenation 2^n b_k is prime, or a(n)=0 if there is no such prime.

1, 1, 1, 1, 1, 4, 1, 1, 2, 3, 1, 1, 4, 4, 6, 30, 3, 1, 6, 1, 32, 3, 3, 2, 22, 1, 6, 1, 2, 14, 7, 1, 10, 1, 2, 6, 3, 4, 2, 5, 2, 6, 1, 1, 37, 53, 53, 13, 64, 1, 67, 1, 45, 29, 17, 12, 14, 1, 2, 5, 15, 36, 10, 7, 1, 1, 81, 4, 18, 5, 55, 8, 33, 19, 8, 6, 2, 11

Offset

0,6

Comments

Conjecture: for all n, a(n)>0.

Links

Table of n, a(n) for n=0..77.

Prog

(PARI) a(n) = {k = 0; while (! isprime(eval(concat(Str(2^n), Str((10^k-1)/3)))), k++); k; } \\ Michel Marcus, Sep 16 2014

Crossrefs

Cf. A000079, A232210, A242775.

Sequence in context: A100261 A370945 A016526 * A030747 A222171 A325529

Adjacent sequences: A247338 A247339 A247340 * A247342 A247343 A247344

Keyword

nonn

Author

Vladimir Shevelev, Sep 14 2014

Extensions

More terms from Michel Marcus, Sep 16 2014

Status

approved