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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A269264 Numbers k such that (2^k-1)^2 - 2 is a semiprime. 1
5, 9, 13, 16, 23, 24, 28, 35, 37, 38, 40, 47, 48, 51, 52, 57, 61, 65, 67, 70, 79, 83, 84, 85, 88, 90, 102, 111, 144, 148, 157, 162, 168, 169, 177, 181, 190, 237, 246, 298, 308, 346 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(43) >= 360. - Hugo Pfoertner, Aug 05 2019

LINKS

Table of n, a(n) for n=1..42.

factordb.com, Status of (2^360-1)^2-2.

EXAMPLE

a(1) = 5 because 31^2-2 = 959 = 7*137 which is semiprime.

a(2) = 9 because 511^2-2 = 261119 = 23*11353 which is semiprime.

MATHEMATICA

Select[Range[120], PrimeOmega[(2^# - 1)^2 - 2] == 2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..120]| IsSemiprime(s) where s is (2^n-1)^2-2];

(PARI) isok(n) = bigomega((2^n-1)^2-2) == 2; \\ Michel Marcus, Feb 22 2016

CROSSREFS

Cf. A091515, A091516, A268574.

Sequence in context: A281582 A190696 A314662 * A314663 A314664 A051541

Adjacent sequences:  A269261 A269262 A269263 * A269265 A269266 A269267

KEYWORD

nonn,more,hard

AUTHOR

Vincenzo Librandi, Feb 21 2016

EXTENSIONS

a(29)-a(42) from Hugo Pfoertner, Aug 05 2019

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 28 17:37 EDT 2020. Contains 334684 sequences. (Running on oeis4.)