%I #27 Sep 08 2022 08:46:08

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

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

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

%C a(19) >= 534. - _Daniel Suteu_, Aug 05 2019

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

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

%o (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..400] | IsSemiprime(s) where s is n*5^n-1];

%Y Cf. similar sequences listed in A242273.

%Y Cf. A059676, A064751.

%K nonn,more

%O 1,2

%A _Vincenzo Librandi_, May 12 2014

%E 1 prepended by _Carl Schildkraut_, Aug 18 2015

%E a(13)-a(17) from _Carl Schildkraut_, Aug 18 2015

%E a(18) from _Daniel Suteu_, Aug 05 2019