A295641 - OEIS

A295641

Smallest k not divisible by 7 such that k*7^n + 1 is prime.

4, 4, 4, 6, 10, 4, 4, 36, 6, 4, 4, 82, 36, 10, 24, 90, 4, 48, 12, 16, 118, 10, 72, 16, 94, 18, 24, 150, 76, 58, 34, 40, 156, 34, 52, 166, 16, 4, 36, 90, 120, 78, 34, 36, 10, 244, 60, 102, 18, 114, 172, 48, 94, 10, 66, 396, 234, 240, 166

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OFFSET

1,1

COMMENTS

The ratio (Sum_(n=1..t) a(n)) / (Sum_(n=1..t) n) tends to log(7) as t increases.

LINKS

Pierre CAMI, Table of n, a(n) for n = 1..2000

MATHEMATICA

Array[Block[{k = 2}, While[Or[Divisible[k, 7], ! PrimeQ[k 7^# + 1]], k++]; k] &, 59] (* Michael De Vlieger, Dec 18 2017 *)

PROG

(PARI) a(n) = {k = 1; while (!isprime(k*7^n+1), k++; if (! (k%7), k++)); k; } \\ Michel Marcus, Nov 25 2017

CROSSREFS

Cf. A057778, A295639, A295640.

Sequence in context: A229630 A141466 A171743 * A171815 A023958 A137751

Adjacent sequences: A295638 A295639 A295640 * A295642 A295643 A295644

KEYWORD

nonn

AUTHOR

Pierre CAMI, Nov 25 2017

STATUS

approved