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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
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
Sequence in context: A123521 A322115 A294217 * A122518 A346031 A129704
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 March 28 05:39 EDT 2024. Contains 371235 sequences. (Running on oeis4.)