A123342 Least m such that 2^n+5^m is prime.

0, 0, 0, 1, 0, 1, 2, 5, 0, 5, 2, 1, 4, 3, 6, 11, 0, 19, 14, 3, 2, 5, 2, 3, 16, 11, 66, 35, 4, 7, 122, 17, 14, 13, 2, 3, 4, 15, 34, 52519, 4, 3, 22, 29, 10, 29, 6, 1, 4, 5, 38

Offset

0,7

Comments

a(51) > 10^5

Links

Table of n, a(n) for n=0..50.

Example

a(0)=0 because 2^0+5^0=1+1=2 is prime.

Mathematica

f[n_] := Block[{p = If[n < 3 || EvenQ@n, 0, 1]}, While[ !PrimeQ[5^p + 2^n ], p += 2 ]; p] ; Table[f@n, {n, 0, 38}] (* Robert G. Wilson v *)

Prog

(PARI) for(n=0, 38, k=2^n; m=0; while(!isprime(k+5^m), m++); print1(m, ", ")) \\ Klaus Brockhaus, Oct 15 2006

Crossrefs

Cf. A123252.

Sequence in context: A067881 A307649 A024714 * A196816 A058204 A242969

Adjacent sequences: A123339 A123340 A123341 * A123343 A123344 A123345

Keyword

nonn,hard,more

Author

Zak Seidov, Oct 11 2006

Extensions

Entries through a(38) checked by Klaus Brockhaus, Oct 15 2006

a(40)-a(50) from Robert G. Wilson v, Nov 14 2006

a(39)=52519 from Max Alekseyev, Mar 15 2011

Status

approved