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

%I

%S 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,

%T 122,17,14,13,2,3,4,15,34,52519,4,3,22,29,10,29,6,1,4,5,38

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

%C a(51) > 10^5

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

%t 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_ *)

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

%Y Cf. A123252.

%K nonn,hard,more

%O 0,7

%A _Zak Seidov_, Oct 11 2006

%E Entries through a(38) checked by _Klaus Brockhaus_, Oct 15 2006

%E a(40)-a(50) from _Robert G. Wilson v_, Nov 14 2006

%E a(39)=52519 from _Max Alekseyev_, Mar 15 2011

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Last modified September 20 08:10 EDT 2021. Contains 347577 sequences. (Running on oeis4.)