A126906 - OEIS

%I #14 Mar 13 2020 13:38:29

%S 1,2,1,2,1,10,17,2,1,2,1,94,122,22,1,80,1,4,6,2,1,242,3,6,5,80,1,12,1,

%T 82,96,2,7,188,1,136,69,158,1,2,1,954,50,118,1,570,14,90,45,6,1,228,

%U 38,4,6,22,1,12,1,580,86,336,24,768,1,1170,408,340,1,896

%N Smallest k such that 1 + k^(2*n+1) + Sum_{j=1..n} k^(2*j) is prime.

%C 1 is a term if and only if number of terms in polynomial is prime.

%H Amiram Eldar, <a href="/A126906/b126906.txt">Table of n, a(n) for n = 1..250</a>

%t a[n_]: = Module[{k = 1}, While[!PrimeQ[1 + k^(2*n+1) + Sum[k^(2*j), {j, 1, n}]], k++]; k]; Array[a, 30] (* _Amiram Eldar_, Mar 13 2020 *)

%o (PARI) a(n) = my(k = 1); while(! isprime(1 + k^(2*n+1) + sum(j=1, n, k^(2*j))), k++); k; \\ _Michel Marcus_, Mar 13 2020

%Y Cf. A124151, A119863, A049407, A124175 A124176 A124177, A124178, A124179, A124180, A124181, A126908-A126916.

%K nonn

%O 1,2

%A _Artur Jasinski_, Dec 31 2006

%E More terms from _Amiram Eldar_, Mar 13 2020