A318875 - OEIS

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A318875

Number of divisors d of n for which 2*phi(d) < d.

4

0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 2, 0, 1, 0, 0, 0, 2, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 3, 0, 0, 0, 1, 0, 4, 0, 1, 0, 3, 0, 3, 0, 2, 0, 1, 0, 4, 0, 2, 0, 2, 0, 3, 0, 3, 0, 1, 0, 6, 0, 1, 0, 0, 0, 3, 0, 2, 0, 3, 0, 6, 0, 1, 0, 2, 0, 3, 0, 4, 0, 1, 0, 6, 0, 1, 0, 3, 0, 5, 0, 2, 0, 1, 0, 5, 0, 2, 0, 4, 0, 3, 0, 3, 1

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OFFSET

1,12

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..65537

FORMULA

a(n) = Sum_{d|n} [A083254(d) < 0].

For all n >= 1, a(n) + A318874(n) + A007814(n) = A000005(n).

MAPLE

A318875 := n -> nops(select(d -> (2*numtheory:-phi(d)) < d, divisors(n))):

seq(A318875(n), n=1..199); # Peter Luschny, Sep 05 2018

MATHEMATICA

A318875[n_] := DivisorSum[n, 1 &, 2*EulerPhi[#] < # &];

Array[A318875, 100] (* Paolo Xausa, Jul 08 2024 *)

PROG

(PARI) A318875(n) = sumdiv(n, d, (2*eulerphi(d))<d);

CROSSREFS

Cf. A000010, A083254, A318874, A318877, A318879.

Sequence in context: A265245 A110270 A284825 * A187143 A187144 A123635

Adjacent sequences: A318872 A318873 A318874 * A318876 A318877 A318878

KEYWORD

nonn

AUTHOR

Antti Karttunen, Sep 05 2018

STATUS

approved