1,2

n is always of the form 4*k + 1

If n is in the sequence and m=2^(n+2)*3*(7*2^n-1) then phi(m)+sigma(m)=3m (m is in the sequence A011251). The proof is easy. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Mar 04 2005

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

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

H. Riesel, ``Prime numbers and computer methods for factorization,'' Progress in Mathematics, Vol. 57, Birkhauser, Boston, 1985, Chap. 4, see pp. 381-384.

H. C. Williams and C. R. Zarnke, Math. Comp., 22 (1968), 420-422.

Wilfrid Keller, List of primes k.2^n - 1 for k < 300

Index entries for sequences of n such that k*2^n-1 (or k*2^n+1) is prime

Do[ If[ PrimeQ[7*2^n - 1], Print[n]], {n, 1, 2500}]

(PARI) v=[ ]; for(n=0, 2000, if(isprime(7*2^n-1), v=concat(v, n), )); v

Cf. A050523, A003307, A002235, A046865, A079906, A046866, A005541, A056725, A046867, A079907.

Cf. A032353, 7*2^n+1 is prime.

Sequence in context: A062777 A102179 A097538 this_sequence A022341 A095725 A005006

Adjacent sequences: A001768 A001769 A001770 this_sequence A001772 A001773 A001774

hard,nonn

N. J. A. Sloane (njas(AT)research.att.com).

More terms from Douglas Burke (dburke(AT)nevada.edu).

More terms from Hugo Pfoertner (hugo(AT)pfoertner.org), Jun 23 2004