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 A180419 #13 Apr 19 2023 09:04:15
%S A180419 1,10,5052,14240070,3152221563324450,157195096511273995860,
%T A180419 2374214683408467590063771983920,618146855974818638210995488847340730,
%U A180419 144946467754033586465978879886385830380958862710
%N A180419 a(n) = (A000111(2*p) - 1)/(2*p), where p = A000040(n) = prime(n).
%C A180419 My comment in A000111 concerning A000111(2*p) mod (2*p) says that all entries are integers.
%t A180419 t = Range[0, 60]! CoefficientList[ Series[ Sec@x + Tan@x, {x, 0, 60}], x]; f[n_] := (Rest[t][[2 Prime@n]] - 1)/(2 Prime@n); Array[f, 9] (* _Robert G. Wilson v_, Sep 04 2010 *)
%o A180419 (Python)
%o A180419 from sympy import euler, prime
%o A180419 def A180419(n): return (1-euler(2*(p:=prime(n))))//p>>1 if n > 1 else 1 # _Chai Wah Wu_, Apr 18 2023
%Y A180419 Cf. A180418, A000111, A180417.
%K A180419 nonn
%O A180419 1,2
%A A180419 _Vladimir Shevelev_, Sep 03 2010
%E A180419 a(6) onwards from _Robert G. Wilson v_, Sep 04 2010
%E A180419 Definition rephrased by _R. J. Mathar_, Sep 29 2010