A252793 Numbers m such that 8^m + m is a semiprime.

1, 3, 5, 7, 11, 15, 21, 25, 75, 107, 221, 257, 273

Offset: 1

Author

Vincenzo Librandi, Dec 25 2014

Keywords

• nonn

Comments

Next terms: 347?, 479, 521, 551, .... - Charles R Greathouse IV, Aug 17 2015

Crossrefs

Cf. similar sequences listed in A252788.

Cf. A252661.

Programs

Magma

IsSemiprime:=func; [m: m in [1..70] | IsSemiprime(s) where s is 8^m+m];

Mathematica

Select[Range[70], PrimeOmega[8^# + #]==2 &]

PARI

is(n)=bigomega(8^n+n)==2 \\ Charles R Greathouse IV, Aug 14 2015

Formula

1 is in this sequence because 8^1+1 = 3*3 is semiprime.

7 is in this sequence because 8^7+7 = 3*699053 and these two factors are prime.

Extensions

a(9)-a(10) from Luke March, Jul 26 2015

a(11) from Carl Schildkraut, Aug 14 2015

a(12)-a(13) from Charles R Greathouse IV, Aug 17 2015