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(17) > 4400000. - Serge Batalov, Jun 20 2023
LINKS
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
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
a(16) from Ryan Propper and Serge Batalov, Jun 20 2023
STATUS
approved