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

Offset: 1

Author

Roger L. Bagula, Oct 07 2006

Keywords

• sign

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

Crossrefs

Cf. A000720, A055670, A103889.

Programs

Mathematica

f[n_] = PrimePi[n] + (-1)^(PrimePi[n] + 1); Table[f[n], {n, 1, 200}]

PARI

a(n) = primepi(n) + (-1)^(primepi(n) + 1); \\ Michel Marcus, Oct 12 2018

Extensions

Edited by N. J. A. Sloane, Oct 08 2006