

A253027


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


2



3, 5, 3, 5, 5, 9, 11, 35, 53, 51, 105, 5, 233, 347, 125, 369, 2063, 89, 4715, 1145, 885, 4839, 2711, 30611, 5859, 2543, 21509, 114071, 309, 60191, 524489, 33305, 306363, 987537, 509765
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,1


LINKS

Table of n, a(n) for n=1..35.


EXAMPLE

3*2^2+1=13 prime so a(1)=3 as A000043(1)=2.
3*2^3+1=25 composite, 5*2^3+1=41 prime so a(2)=5 as A000043(2)=3.
3*2^5+1=97 prime so a(3)=3 as A000043(3)=5.


MATHEMATICA

a253027[n_] :=
Block[{k, t = Select[Prime[Range[n]], PrimeQ[2^#  1] &], l},
l = Length[t];
Table[k = 3; While[! PrimeQ[k*2^t[[i]] + 1], k = k + 2]; k, {i, l}]]; a253027[600] (* Michael De Vlieger, Dec 26 2014 *)


PROG

(PFGW)
Command pfgw64 f e1000000 in.txt
in.txt file :
ABC2 a$*2^756839+$b // {number_primes, $b, 1}
b: from 1 to 1
a: from 1 to 1000000
(PARI) lista(nn) = {forprime (n=1, nn, if (isprime(2^n1), k=3; while (!isprime(k*2^n+1), k += 2); print1(k, ", "); ); ); } \\ Michel Marcus, Dec 27 2014


CROSSREFS

Cf. A000043, A135434.
Sequence in context: A020765 A112756 A121795 * A249384 A228446 A188889
Adjacent sequences: A253024 A253025 A253026 * A253028 A253029 A253030


KEYWORD

nonn,more


AUTHOR

Pierre CAMI, Dec 26 2014


EXTENSIONS

a(33)a(35) from Pierre CAMI, Apr 06 2015


STATUS

approved



