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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

Table of n, a(n) for n=1..18.

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

N. J. A. Sloane.

EXTENSIONS

More terms from Sean A. Irvine, May 04 2017

STATUS

approved

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Last modified March 18 22:11 EDT 2019. Contains 321305 sequences. (Running on oeis4.)