The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A092561 "Mersenne" semiprimes, semiprimes of the form 2^n-1. 7
 15, 511, 2047, 8388607, 137438953471, 2199023255551, 562949953421311, 576460752303423487, 147573952589676412927, 9671406556917033397649407, 158456325028528675187087900671, 2535301200456458802993406410751 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE 2047 is a member because 2047 = 2^11 - 1 = 23*89. MATHEMATICA a := Select[Range[2, 120], ! PrimeQ[2^# - 1] && Length[Divisors[2^# - 1]] <= 4 &]; 2^a - 1 (* Stefan Steinerberger, Apr 12 2006 *) Select[2^Range[0, 110]-1, PrimeOmega[#] == 2&] (* Harvey P. Dale, Feb 22 2013 *) CROSSREFS Corresponding n: A085724. Cf. A092558, A092559 and A092562. Sequence in context: A218188 A218365 A203326 * A218312 A218280 A218632 Adjacent sequences:  A092558 A092559 A092560 * A092562 A092563 A092564 KEYWORD nonn AUTHOR Zak Seidov, Feb 27 2004 EXTENSIONS One more term from Stefan Steinerberger, Apr 12 2006 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified April 17 11:26 EDT 2021. Contains 343064 sequences. (Running on oeis4.)