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 A123246 #15 Oct 13 2018 09:08:57
%S A123246 -1,2,1,1,4,4,3,3,3,3,6,6,5,5,5,5,8,8,7,7,7,7,10,10,10,10,10,10,9,9,
%T A123246 12,12,12,12,12,12,11,11,11,11,14,14,13,13,13,13,16,16,16,16,16,16,15,
%U A123246 15,15,15,15,15,18,18,17,17,17,17,17,17,20,20,20,20,19,19,22,22,22,22,22,22,21,21,21,21,24,24,24,24,24,24,23,23,23,23
%N A123246 a(n) = PrimePi(n) + (-1)^(PrimePi(n) + 1) (cf. A000720).
%D A123246 Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology, Dover, New York, 1978, page 134.
%H A123246 G. C. Greubel, <a href="/A123246/b123246.txt">Table of n, a(n) for n = 1..10000</a>
%t A123246 f[n_] = PrimePi[n] + (-1)^(PrimePi[n] + 1); Table[f[n], {n, 1, 200}]
%o A123246 (PARI) a(n) = primepi(n) + (-1)^(primepi(n) + 1); \\ _Michel Marcus_, Oct 12 2018
%Y A123246 Cf. A000720, A055670, A103889.
%K A123246 sign
%O A123246 1,2
%A A123246 _Roger L. Bagula_, Oct 07 2006
%E A123246 Edited by _N. J. A. Sloane_, Oct 08 2006