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A125739 Primes p such that 3^p + 3^((p + 1)/2) + 1 is prime. 3
3, 5, 7, 17, 19, 79, 163, 317, 353, 1049, 1759, 5153, 7541, 23743, 2237561 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

PrimePi[ a(n) ] = {2, 3, 4, 7, 8, 22, 38, 66, 71, 176, 274, 687, 956, ...}, the indices of the primes p.

a(16) > 2300000. - Serge Batalov, Oct 12 2014

LINKS

Table of n, a(n) for n=1..15.

J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.

S. S. Wagstaff, Jr., The Cunningham Project.

MATHEMATICA

Do[p=Prime[n]; f=3^p+3^((p+1)/2)+1; If[PrimeQ[f], Print[{n, p}]], {n, 1, 200}]

PROG

(PARI) lista(nn) = {forprime(p=3, nn, if (ispseudoprime(3^p + 3^((p + 1)/2) + 1), print1(p, ", ")); ); } \\ Michel Marcus, Oct 13 2014

(MAGMA) [p: p in PrimesUpTo(5000) | IsPrime(3^p+3^((p+1)div 2)+1)]; // Vincenzo Librandi, Oct 13 2014

CROSSREFS

Cf. A125738 = Primes p such that 3^p - 3^((p + 1)/2) + 1 is prime.

Cf. A007670 = Numbers n such that 2^n - 2^((n + 1)/2) + 1 is prime.

Cf. A007671 = Numbers n such that 2^n + 2^((n + 1)/2) + 1 is prime.

Cf. A066408 = Numbers n such that the Eisenstein integer has prime norm.

Sequence in context: A087126 A331800 A062547 * A219461 A122853 A137258

Adjacent sequences:  A125736 A125737 A125738 * A125740 A125741 A125742

KEYWORD

hard,more,nonn

AUTHOR

Alexander Adamchuk, Dec 02 2006

EXTENSIONS

a(11)-a(13) from Stefan Steinerberger, Sep 08 2007

a(14) from Lelio R Paula (lelio(AT)sknet.com.br), May 07 2008

a(15) from Serge Batalov, Oct 12 2014

STATUS

approved

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Last modified August 14 19:09 EDT 2020. Contains 336483 sequences. (Running on oeis4.)