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 (mymontain(AT)yahoo.com), Feb 09 2005

Salas shows that primes whose reciprocals are in the Cantor set are precisely those of the form (3^a(n) - 1)/2. - Charles R Greathouse IV Jul 29 2009

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

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, Repunit

H. Lifchitz, Mersenne and Fermat primes field

Christian Salas, Base-3 repunit primes and the Cantor set

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

Cf. A076481, A033632.

Adjacent sequences: A028488 A028489 A028490 this_sequence A028492 A028493 A028494

Sequence in context: A103564 A083201 A004060 this_sequence A137474 A071087 A038691

nonn

N. J. A. Sloane (njas(AT)research.att.com), Jean-Yves Perrier (nperrj(AT)ascom.ch)

36913 from Farideh Firoozbakht (mymontain(AT)yahoo.com), Mar 27 2005

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