A252662 Numbers m such that 9^m - m is a semiprime.

4, 8, 10, 14, 20, 26, 38, 44, 56, 80, 190

Offset

1,1

Comments

a(12) >= 250. - Hugo Pfoertner, Aug 03 2019

Links

Table of n, a(n) for n=1..11.

factordb.com, Status of 9^250-250.

Example

4 is in this sequence because 9^4-4 = 79*83 and these two factors are prime.
14 is in this sequence because 9^14-14 = 163*140348419969 and these two factors are prime.

Mathematica

Select[Range[80], PrimeOmega[9^# - #]==2 &]

Prog

(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..80] | IsSemiprime(s) where s is 9^m-m];

Crossrefs

Cf. similar sequences listed in A252656.

Sequence in context: A310995 A310996 A028873 * A310997 A310998 A153975

Adjacent sequences: A252659 A252660 A252661 * A252663 A252664 A252665

Keyword

nonn,more,hard

Author

Vincenzo Librandi, Dec 22 2014

Extensions

a(11) from Hugo Pfoertner, Aug 03 2019

Status

approved