A302387 a(n) is least number k >= 3 such that (k^(2^n) + (k-2)^(2^n))/2 is prime.

3, 3, 3, 5, 3, 3, 3, 179, 169, 935, 663, 8723, 1481, 2035, 10199, 18203, 36395

Offset

0,1

Links

Table of n, a(n) for n=0..16.

Henri Lifchitz & Renaud Lifchitz, (36395^65536+36393^65536)/2, a(16).

Example

a(10)=663 corresponds to the prime (663^1024 + 661^1024)/2.

Mathematica

lst = {};  For[n=0, n<=14, n++, k=3;  While[! PrimeQ[(k^(2^n) + (k-2)^(2^n))/2], k++];  AppendTo[lst, k]];  lst (* Robert Price, Apr 29 2018 *)

Prog

(PARI) for(n=0, 20, forstep(k=3, +oo, 2, if(ispseudoprime((k^(2^n)+(k-2)^(2^n))/2), print1(k, ", "); break())))

Crossrefs

Cf. A253633, A291944, A301738.

Sequence in context: A242033 A375848 A301738 * A049613 A002373 A236569

Adjacent sequences: A302384 A302385 A302386 * A302388 A302389 A302390

Keyword

nonn,hard,more

Author

Jeppe Stig Nielsen, Apr 06 2018

Extensions

a(15) from Robert Price, May 28 2018

a(16) from Kellen Shenton, Apr 14 2022

Status

approved