A153441 - OEIS

A153441

Numbers k such that k^27*(k^27+1)+1 is prime.

1, 21, 50, 77, 153, 191, 194, 311, 405, 440, 462, 557, 638, 659, 690, 746, 852, 887, 944, 945, 1140, 1326, 1344, 1452, 1463, 1607, 1632, 1652, 1659, 1683, 1710, 1788, 1812, 1851, 1925, 1943, 1992, 2157, 2294, 2309, 2352, 2402, 2621, 2687, 2700, 2733, 2756

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OFFSET

1,2

COMMENTS

It seems numbers of the form k^n*(k^n+1)+1 with n > 0, k > 1 may be primes only if n has the form 3^j. When n is even, k^(4*n)+k^(2*n)+1=(k^(2*n)+1)^2-(k^n)^2=(k^(2*n)+k^n+1)*(k^(2*n)-k^n+1) so composite. But why if n odd > 3 and not a power of 3, k^n*(k^n+1)+1 is always composite ??

LINKS

Pierre CAMI, Table of n, a(n) for n=1,...,37719

PROG

(PARI) isok(k) = isprime(k^27*(k^27+1)+1); \\ Michel Marcus, Sep 20 2019

CROSSREFS

Cf. A153438.

Sequence in context: A141556 A147281 A130062 * A357679 A235884 A053178

Adjacent sequences: A153438 A153439 A153440 * A153442 A153443 A153444

KEYWORD

nonn

AUTHOR

Pierre CAMI, Dec 26 2008

STATUS

approved