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

Author

Zak Seidov, Oct 11 2006

Keywords

• nonn
• hard
• more

Comments

a(51) > 10^5

Examples

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

Crossrefs

Cf. A123252.

Programs

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 *)

PARI

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