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A006515
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Mersenne numbers with at most 2 prime factors.
(Formerly M2695)
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1
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3, 7, 31, 127, 2047, 8191, 131071, 524287, 8388607, 2147483647, 137438953471, 2199023255551, 576460752303423487, 2305843009213693951, 147573952589676412927, 9671406556917033397649407, 618970019642690137449562111, 158456325028528675187087900671
(list;
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refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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REFERENCES
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John Brillhart, D. H. Lehmer, J. L. Selfridge, Bryant Tuckerman and S. S. Wagstaff, Jr., 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).
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LINKS
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John Brillhart, D. H. Lehmer, J. L. Selfridge, Bryant Tuckerman and S. S. Wagstaff, Jr., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
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FORMULA
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MAPLE
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i := 2^(ithprime(n))-1:
if (nops(numtheory[factorset](i)) < 3) then
return i
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MATHEMATICA
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Select[Array[2^Prime[#] - 1 &, 25], PrimeOmega@ # <= 2 &] (* Michael De Vlieger, May 04 2017 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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