|
|
A252663
|
|
Numbers m such that 10^m - m is a semiprime.
|
|
2
|
|
|
1, 7, 9, 11, 15, 33, 77, 93, 107, 117, 143, 149, 177, 209, 221
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,2
|
|
COMMENTS
|
|
|
LINKS
|
|
|
EXAMPLE
|
1 is in this sequence because 10^1-1 = 3*3 is semiprime.
9 is in this sequence because 10^9-9 = 67*14925373 and these two factors are prime.
|
|
MATHEMATICA
|
Select[Range[80], PrimeOmega[10^# - #]==2 &]
|
|
PROG
|
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..80] | IsSemiprime(s) where s is 10^m-m];
(PARI) is(m) = bigomega(10^m - m) == 2; \\ Jinyuan Wang, Jul 09 2019
|
|
CROSSREFS
|
Cf. similar sequences listed in A252656.
|
|
KEYWORD
|
nonn,more,hard
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|