A123246 - OEIS

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A123246

a(n) = PrimePi(n) + (-1)^(PrimePi(n) + 1) (cf. A000720).

-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