cp's OEIS Frontend

A253027 Smallest odd number k>1 such that k*2^A000043(n)+1 is a prime number.

%I A253027 #16 Apr 07 2015 11:31:21
%S A253027 3,5,3,5,5,9,11,35,53,51,105,5,233,347,125,369,2063,89,4715,1145,885,
%T A253027 4839,2711,30611,5859,2543,21509,114071,309,60191,524489,33305,306363,
%U A253027 987537,509765
%N A253027 Smallest odd number k>1 such that k*2^A000043(n)+1 is a prime number.
%e A253027 3*2^2+1=13 prime so a(1)=3 as A000043(1)=2.
%e A253027 3*2^3+1=25 composite, 5*2^3+1=41 prime so a(2)=5 as A000043(2)=3.
%e A253027 3*2^5+1=97 prime so a(3)=3 as A000043(3)=5.
%t A253027 a253027[n_] :=
%t A253027 Block[{k, t = Select[Prime[Range[n]], PrimeQ[2^# - 1] &], l},
%t A253027   l = Length[t];
%t A253027 Table[k = 3; While[! PrimeQ[k*2^t[[i]] + 1], k = k + 2]; k, {i, l}]]; a253027[600] (* _Michael De Vlieger_, Dec 26 2014 *)
%o A253027 (PFGW)
%o A253027 Command pfgw64 -f -e1000000 in.txt
%o A253027 in.txt file :
%o A253027 ABC2 a$*2^756839+$b // {number_primes,$b,1}
%o A253027 b: from 1 to 1
%o A253027 a: from 1 to 1000000
%o A253027 (PARI) lista(nn) = {forprime (n=1, nn, if (isprime(2^n-1), k=3; while (!isprime(k*2^n+1), k += 2); print1(k, ", ");););} \\ _Michel Marcus_, Dec 27 2014
%Y A253027 Cf. A000043, A135434.
%K A253027 nonn,more
%O A253027 1,1
%A A253027 _Pierre CAMI_, Dec 26 2014
%E A253027 a(33)-a(35) from _Pierre CAMI_, Apr 06 2015