1,1

If n is in the sequence and m=3^(n-1) then m is a term of A033632 (phi(sigma(m)) = sigma(phi(m)), so 3^(A028491-1) is a subsequence of A033632. For example since 9551 is in the sequence, phi(sigma(3^9550)) = sigma(phi(3^9550)). - Farideh Firoozbakht (f.firoozbakht(AT)math.ui.ac.ir), Feb 09 2005

J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.

H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927-930.

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

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

H. Lifchitz, Mersenne and Fermat primes field

Do[If[PrimeQ[(3^n-1)/2], Print[n]], {n, 10000}] (Firoozbakht)

Cf. A033632.

Sequence in context: A103564 A083201 A004060 this_sequence A137474 A071087 A038691

Adjacent sequences: A028488 A028489 A028490 this_sequence A028492 A028493 A028494

nonn

njas, Jean-Yves Perrier (nperrj(AT)ascom.ch)

36913 from Farideh Firoozbakht (f.firoozbakht(AT)math.ui.ac.ir), Mar 27 2005

a(14), a(15) & a(16) from Robert G. Wilson v (rgwv(AT)rgwv.com), Apr 11 2005