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A242273 Numbers n such that n*2^n - 1 is a semiprime. 11
5, 7, 8, 9, 10, 12, 18, 20, 25, 32, 37, 39, 72, 80, 85, 90, 97, 142, 150, 159, 163, 168, 169, 186, 192, 211, 231, 272, 305, 349, 363, 369, 375, 463, 465, 615, 668, 672, 789, 797, 817, 859, 908, 938, 951, 1092, 1123 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The semiprimes of this form are: 159, 895, 2047, 4607, 10239, ... (A242115).

a(48) >= 1152. - Hugo Pfoertner, Jul 29 2019

LINKS

Table of n, a(n) for n=1..47.

FactorDB, Status of 1152*2^1152-1.

FORMULA

A003261(a(n)) = A242115(n). - Amiram Eldar, Nov 27 2019

MATHEMATICA

Select[Range[1000], PrimeOmega[# 2^# - 1]==2&]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..1000] | IsSemiprime(s) where s is n*2^n-1];

CROSSREFS

Cf. numbers n such that n*k^n - 1 is semiprime: this sequence (k=2), A242274 (k=3), A242335 (k=4), A242336 (k=5), A242337 (k=6), A242338 (k=7), A242339 (k=8), A242340 (k=9), A242341 (k=10).

Cf. A002234, A003261, A242115, A242175.

Sequence in context: A078892 A164374 A072281 * A261917 A111339 A273786

Adjacent sequences:  A242270 A242271 A242272 * A242274 A242275 A242276

KEYWORD

nonn,more,hard

AUTHOR

Vincenzo Librandi, May 12 2014

EXTENSIONS

a(28)-a(29) from Luke March, Aug 05 2015

a(30)-a(42) from Carl Schildkraut, Aug 18 2015

Corrected and extended by Luke March, Sep 01 2015

Missing terms a(26)-a(27) inserted by Amiram Eldar, Nov 27 2019

STATUS

approved

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Last modified July 9 14:56 EDT 2020. Contains 335543 sequences. (Running on oeis4.)