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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^n-1), 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

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Last modified December 11 18:19 EST 2019. Contains 329925 sequences. (Running on oeis4.)