This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A242336 #27 Sep 08 2022 08:46:08 %S A242336 1,2,6,12,15,19,20,26,50,55,66,68,96,99,150,166,228,459 %N A242336 Numbers k such that k*5^k-1 is semiprime. %C A242336 The semiprimes of this form are: 4, 49, 93749, 2929687499, 457763671874, 362396240234374, 1907348632812499, 38743019104003906249, 4440892098500626161694526672363281249, 15265566588595902430824935436248779296874, ... %C A242336 a(19) >= 534. - _Daniel Suteu_, Aug 05 2019 %p A242336 select(t -> (numtheory:-bigomega(t*5^t-1)=2), [$1..400]); # _Robert Israel_, Aug 18 2015 %t A242336 Select[Range[400], PrimeOmega[# 5^# - 1]==2&] %o A242336 (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 A242336 Cf. similar sequences listed in A242273. %Y A242336 Cf. A059676, A064751. %K A242336 nonn,more %O A242336 1,2 %A A242336 _Vincenzo Librandi_, May 12 2014 %E A242336 1 prepended by _Carl Schildkraut_, Aug 18 2015 %E A242336 a(13)-a(17) from _Carl Schildkraut_, Aug 18 2015 %E A242336 a(18) from _Daniel Suteu_, Aug 05 2019