A055670 -id:A055670 - cp's OEIS frontend

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

Roger L. Bagula, Oct 07 2006

sign

Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology, Dover, New York, 1978, page 134.

G. C. Greubel, Table of n, a(n) for n = 1..10000

Cf. A000720, A055670, A103889.

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

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

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

A194264 Sum of divisors of all elements of the n-th set of consecutive integers that are also nonprimes.

Original entry on oeis.org

1, 7, 12, 46, 28, 79, 39, 110, 229, 72, 304, 206, 96, 234, 444, 482, 168, 555, 366, 195, 642, 433, 764, 1158, 544, 216, 564, 280, 616, 2639, 683, 1210, 288, 2211, 372, 1406, 1389, 834, 1555, 1490, 546, 2788, 508, 1029, 468, 3913, 3948, 1249, 560, 1266, 2006

Offset: 1

Omar E. Pol, Sep 03 2011

			46 is in the sequence because the fourth set of consecutive natural numbers that are also nonprimes is {8, 9, 10}, the sum of divisors of 8 is 1+2+4+8=15, the sum of divisors of 9 is 1+3+9=13, the sum of divisors of 10 is 1+2+5+10=18, so a(4) = 15+13+18 = 46.

Alois P. Heinz, Table of n, a(n) for n = 1..10000

Cf. A000203, A018252, A054265, A055670.

with(numtheory):
a:= n-> `if`(n=1, 1, add(sigma(i), i=ithprime(n)+1..ithprime(n+1)-1)):
seq(a(n), n=1..80);  # Alois P. Heinz, Oct 18 2011

Table[Plus@@Flatten[Divisors[Range[Prime[n] - (-1)^Prime[n], Prime[n + 1] + (-1)^Prime[n + 1]]]], {n, 2, 50}] (* Alonso del Arte, Oct 18 2011 *)

cp's OEIS Frontend

A123246 a(n) = PrimePi(n) + (-1)^(PrimePi(n) + 1) (cf. A000720).

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