1,2

There are no other known terms under 1000 - the only possible terms are 812 and 908. - Carl Schildkraut, Aug 21 2015

Table of n, a(n) for n=1..17.

1 is in this sequence because 5^1-1 = 2*2 is semiprime.

18 is in this sequence because 5^18-18 = 199*19169332993 and these two factors are prime.

Select[Range[100], PrimeOmega[5^# - #]==2 &]

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..100] | IsSemiprime(s) where s is 5^m-m];

(PARI) isok(n)=bigomega(5^n - n)==2 \\ Anders HellstrÃ¶m, Aug 21 2015

Cf. similar sequences listed in A252656.

Sequence in context: A063218 A104656 A179993 * A140492 A184517 A028252

Adjacent sequences: A252655 A252656 A252657 * A252659 A252660 A252661

nonn,more

Vincenzo Librandi, Dec 20 2014

a(13)-a(17) from Carl Schildkraut, Aug 21 2015

approved