A101221 Least k such that (10^k)*(P(n)#) - 1 is prime with P(i)= i-th prime and P(i)#= i-th primorial.

1, 1, 2, 1, 1, 1, 3, 2, 1, 1, 40, 1, 1, 10, 10, 10, 7, 17, 4, 11, 15, 10, 11, 54, 24, 8, 11, 2, 18, 2, 6, 4, 58, 3, 14, 15, 1, 3, 13, 16, 1, 37, 4, 4, 8, 11, 80, 2, 37, 58, 55, 34, 15, 59, 26, 1, 32, 42, 47, 56, 47, 4, 57, 62, 27, 18, 14, 17, 12, 47, 19, 16, 39, 9, 71, 13, 6, 6, 25, 10, 1, 1

Offset

1,3

Links

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

Example

(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.

Prog

(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

Crossrefs

Cf. A002110.

Sequence in context: A183214 A346009 A344756 * A086291 A262766 A262747

Adjacent sequences: A101218 A101219 A101220 * A101222 A101223 A101224

Keyword

nonn

Author

Pierre CAMI, Dec 15 2004

Extensions

Corrected by Michel Marcus, Aug 06 2014

Status

approved