A343120 - OEIS

A343120

a(n) is the smallest number b such that (b^(2^k) + 1)/2 is prime for k = 0, 1, ..., n.

3, 3, 3, 205, 2326161, 20589460461, 3847314721101

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OFFSET

0,1

LINKS

Table of n, a(n) for n=0..6.

EXAMPLE

For n=3, the four numbers (205+1)/2, (205^2+1)/2, (205^4+1)/2, and (205^8+1)/2 are prime, and 205 is smallest with this property, so a(3)=205.

PROG

(PARI) a(n)=forstep(b=3, +oo, 2, for(k=0, n, !ispseudoprime((b^(2^k)+1)/2)&&next(2)); return(b)) \\ if a(n-1) is known, b loop can start from there instead

CROSSREFS

Cf. A090872, A275530.

Sequence in context: A217671 A118539 A015665 * A230667 A131462 A230811

Adjacent sequences: A343117 A343118 A343119 * A343121 A343122 A343123

KEYWORD

nonn,more,hard

AUTHOR

Jeppe Stig Nielsen, Apr 05 2021

EXTENSIONS

a(6) found by Kellen Shenton added by Jeppe Stig Nielsen, Apr 09 2021

STATUS

approved