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
FORMULA
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
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