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 A099442 #24 Apr 26 2021 14:43:01
%S A099442 4,26,57,247,502,4083,1073741793,4294967263,8589934558,70368744177617,
%T A099442 4503599627370443,4611686018427387841,18889465931478580854709,
%U A099442 75557863725914323419059,77371252455336267181195177,316912650057057350374175801245
%N A099442 Semiprimes of the form 2^k-k-1.
%C A099442 Semiprimes in A000295.
%H A099442 Hugo Pfoertner, <a href="/A099442/b099442.txt">Table of n, a(n) for n = 1..31</a> (first 19 terms from Vincenzo Librandi)
%e A099442 a(4) = 247 because 247 = 13*19 = 2^8-8-1 = 2^A099441(4)-A099441(4)-1.
%t A099442 Select[Table[2^n - n - 1, {n, 300}], PrimeOmega[#] == 2&] (* _Vincenzo Librandi_, Sep 21 2012 *)
%o A099442 (Python)
%o A099442 from sympy.ntheory.factor_ import primeomega
%o A099442 def ok(n): return primeomega(2**n-n-1) == 2
%o A099442 print([2**m-m-1 for m in range(2, 100) if ok(m)]) # _Michael S. Branicky_, Apr 26 2021
%Y A099442 Cf. A099439, A099440, A099441.
%K A099442 nonn
%O A099442 1,1
%A A099442 _Hugo Pfoertner_, Oct 18 2004