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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
OFFSET
1,2
COMMENTS
a(16) >= 269. - Hugo Pfoertner, Aug 04 2019
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.
Sequence in context: A254937 A222947 A107226 * A317633 A259045 A029611
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, Dec 22 2014
EXTENSIONS
a(8) from Luke March, Jul 08 2015
a(9)-a(13) from Daniel Suteu, Jul 09 2019
a(14), a(15) from Hugo Pfoertner, Aug 04 2019
STATUS
approved