A113933 a(n) is the least k such that 2^2^n + 1 - 10^k = A000215(n) - 10^k is a (positive) prime, or -1 if no such k exists.

0, -1, 1, 2, 2, 2, -1, 16, 28, 6, 30, 6, 130, -1, 255, 6795, 2670, 1006

Offset

0,4

Links

Table of n, a(n) for n=0..17.

Example

F5 - 10^2 = 4294967197 is prime.

Prog

(PARI) a(n) = my(x=2^2^n); for(k=0, oo, if(ispseudoprime(x), return(k)); x-=9*10^k; if(x<=0, return(-1))) \\ Jianing Song, May 14 2026

Crossrefs

Cf. A000215.

Sequence in context: A306707 A180958 A295854 * A230630 A343957 A338505

Adjacent sequences: A113930 A113931 A113932 * A113934 A113935 A113936

Keyword

sign,hard,more,less

Author

Joao Carlos Leandro da Silva (zxawyh66(AT)yahoo.com), Jan 30 2006

Extensions

Edited by Jianing Song, May 14 2026

a(15)-a(17) from Michael S. Branicky, May 28 2026

Status

approved