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 A123438 #22 Dec 16 2024 11:34:38
%S A123438 4,5,6,7,10,11,15,16,34,44,63,66,168,427,575,928,1094,1218,1363,1713,
%T A123438 5278,10814
%N A123438 Numbers k such that the denominator of k!/!k (= A000142(k)/A000166(k)) is prime.
%C A123438 That is, numbers k such that A053557(k) is prime. - _Michel Marcus_, Aug 28 2013
%t A123438 a[n_] := Sum[(-1)^k/k!, {k, 0, n}]; Select[Range[100], PrimeQ[Numerator[a[#]]] &] (* _G. C. Greubel_, Oct 31 2017 *)
%o A123438 (PARI) isok(n) = isprime(numerator(sum(k=0, n, (-1)^k/k!))); \\ _Michel Marcus_, Aug 28 2013
%Y A123438 Cf. A000142, A000166, A053557.
%K A123438 nonn,more
%O A123438 1,1
%A A123438 _Ed Pegg Jr_, Jul 11 2008
%E A123438 a(16)-a(20) from _G. C. Greubel_, Nov 01 2017
%E A123438 a(21)-a(22) from _Michael S. Branicky_, Dec 16 2024