A252663 Numbers m such that 10^m - m is a semiprime.

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

Links

Table of n, a(n) for n=1..15.

factorbd.com, Status of 10^269-269.

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

Adjacent sequences: A252660 A252661 A252662 * A252664 A252665 A252666

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