1,3

Table of n, a(n) for n=1..82.

(10^1)*2 -1 = 19 prime so k(1)=1.

(10^1)*6 -1 = 59 prime so k(2)=1.

(10^2)*30 -1 = 2999 prime so k(3)=2.

(PARI) a(n) = {prd = prod(i=1, n, prime(i)); k = 1; while (!isprime(10^k*prd-1), k++); k; } \\ Michel Marcus, Aug 06 2014

Cf. A002110.

Sequence in context: A316978 A284345 A183214 * A086291 A262766 A262747

Adjacent sequences: A101218 A101219 A101220 * A101222 A101223 A101224

nonn

Pierre CAMI, Dec 15 2004

Corrected by Michel Marcus, Aug 06 2014

approved