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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.)