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 A123246 a(n) = PrimePi(n) + (-1)^(PrimePi(n) + 1) (cf. A000720). 1
 -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, 12, 12, 12, 12, 12, 12, 11, 11, 11, 11, 14, 14, 13, 13, 13, 13, 16, 16, 16, 16, 16, 16, 15, 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology, Dover, New York, 1978, page 134. LINKS G. C. Greubel, Table of n, a(n) for n = 1..10000 MATHEMATICA f[n_] = PrimePi[n] + (-1)^(PrimePi[n] + 1); Table[f[n], {n, 1, 200}] PROG (PARI) a(n) = primepi(n) + (-1)^(primepi(n) + 1); \\ Michel Marcus, Oct 12 2018 CROSSREFS Cf. A000720, A055670, A103889. Sequence in context: A123521 A322115 A294217 * A122518 A129704 A144460 Adjacent sequences:  A123243 A123244 A123245 * A123247 A123248 A123249 KEYWORD sign AUTHOR Roger L. Bagula, Oct 07 2006 EXTENSIONS Edited by N. J. A. Sloane, Oct 08 2006 STATUS approved

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Last modified August 10 22:22 EDT 2020. Contains 336403 sequences. (Running on oeis4.)