A242336 Numbers k such that k*5^k-1 is semiprime.

1, 2, 6, 12, 15, 19, 20, 26, 50, 55, 66, 68, 96, 99, 150, 166, 228, 459

Offset: 1

Author

Vincenzo Librandi, May 12 2014

Keywords

• nonn
• more

Comments

The semiprimes of this form are: 4, 49, 93749, 2929687499, 457763671874, 362396240234374, 1907348632812499, 38743019104003906249, 4440892098500626161694526672363281249, 15265566588595902430824935436248779296874, ...

a(19) >= 534. - Daniel Suteu, Aug 05 2019

Crossrefs

Cf. similar sequences listed in A242273.

Cf. A059676, A064751.

Programs

Magma

IsSemiprime:=func; [n: n in [1..400] | IsSemiprime(s) where s is n*5^n-1];

Maple

select(t -> (numtheory:-bigomega(t*5^t-1)=2), [$1..400]); # Robert Israel, Aug 18 2015

Mathematica

Select[Range[400], PrimeOmega[# 5^# - 1]==2&]

Extensions

1 prepended by Carl Schildkraut, Aug 18 2015

a(13)-a(17) from Carl Schildkraut, Aug 18 2015

a(18) from Daniel Suteu, Aug 05 2019