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 A006515 Mersenne numbers with at most 2 prime factors. (Formerly M2695) 1
 3, 7, 31, 127, 2047, 8191, 131071, 524287, 8388607, 2147483647, 137438953471, 2199023255551, 576460752303423487, 2305843009213693951, 147573952589676412927, 9671406556917033397649407, 618970019642690137449562111, 158456325028528675187087900671 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002. C. K. Caldwell, Mersenne primes S. S. Wagstaff, Jr., The Cunningham Project MAPLE A006515 := proc(n) local i; i := 2^(ithprime(n))-1: if (nops(numtheory[factorset](i)) < 3) then    return i fi: end: seq(A006515(n), n=1..19); MATHEMATICA Select[Array[2^Prime[#] - 1 &, 25], PrimeOmega@ # <= 2 &] (* Michael De Vlieger, May 04 2017 *) CROSSREFS Cf. A001348. Sequence in context: A105768 A084924 A001348 * A093535 A081093 A057612 Adjacent sequences:  A006512 A006513 A006514 * A006516 A006517 A006518 KEYWORD nonn AUTHOR EXTENSIONS More terms from Sean A. Irvine, May 04 2017 STATUS approved

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Last modified April 14 07:59 EDT 2021. Contains 342946 sequences. (Running on oeis4.)