A252658 Numbers m such that 5^m - m is a semiprime.

1, 3, 8, 14, 18, 48, 52, 54, 62, 72, 96, 98, 114, 186, 486, 524, 712

Offset: 1

Author

Vincenzo Librandi, Dec 20 2014

Keywords

• nonn
• more

Comments

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

Examples

			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.
		

Crossrefs

Cf. similar sequences listed in A252656.

Programs

Magma

IsSemiprime:=func; [m: m in [1..100] | IsSemiprime(s) where s is 5^m-m];

Mathematica

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

PARI

isok(n)=bigomega(5^n - n)==2 \\ Anders Hellström, Aug 21 2015

Extensions

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