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 A001771 Numbers n such that 7*2^n - 1 is prime. (Formerly M3784 N1541) 17
 1, 5, 9, 17, 21, 29, 45, 177, 18381, 22529, 24557, 26109, 34857, 41957, 67421, 70209, 169085, 173489, 177977, 363929, 372897 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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, Mar 04 2005 REFERENCES H. Riesel, ``Prime numbers and computer methods for factorization,'' Progress in Mathematics, Vol. 57, Birkhauser, Boston, 1985, Chap. 4, see pp. 381-384. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). H. C. Williams and C. R. Zarnke, Math. Comp., 22 (1968), 420-422. LINKS Wilfrid Keller, List of primes k.2^n - 1 for k < 300 MATHEMATICA Do[ If[ PrimeQ[7*2^n - 1], Print[n]], {n, 1, 2500}] PROG (PARI) v=[ ]; for(n=0, 2000, if(isprime(7*2^n-1), v=concat(v, n), )); v CROSSREFS Cf. A050523, A003307, A002235, A046865, A079906, A046866, A005541, A056725, A046867, A079907. Cf. A032353, 7*2^n+1 is prime. Sequence in context: A273762 A273851 A097538 * A288448 A022341 A255651 Adjacent sequences:  A001768 A001769 A001770 * A001772 A001773 A001774 KEYWORD hard,nonn AUTHOR EXTENSIONS More terms from Douglas Burke (dburke(AT)nevada.edu). More terms from Hugo Pfoertner, Jun 23 2004 STATUS approved

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Last modified January 23 22:16 EST 2020. Contains 331177 sequences. (Running on oeis4.)