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