

A123342


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


2



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
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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



