|
| |
|
|
A242273
|
|
Numbers n such that n*2^n - 1 is a semiprime.
|
|
12
|
|
|
|
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).
|
|
|
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
|
|
|
|
EXTENSIONS
|
Missing terms a(26)-a(27) inserted by Amiram Eldar, Nov 27 2019
|
|
|
STATUS
|
approved
|
| |
|
|
