A348985 - OEIS

A348985

Numerator of ratio sigma(n) / A325973(n), where A325973 is the arithmetic mean of {sum of squarefree divisors} and {sum of unitary divisors}.

1, 1, 1, 7, 1, 1, 1, 5, 13, 1, 1, 7, 1, 1, 1, 31, 1, 13, 1, 7, 1, 1, 1, 5, 31, 1, 5, 7, 1, 1, 1, 7, 1, 1, 1, 91, 1, 1, 1, 5, 1, 1, 1, 7, 13, 1, 1, 31, 57, 31, 1, 7, 1, 5, 1, 5, 1, 1, 1, 7, 1, 1, 13, 127, 1, 1, 1, 7, 1, 1, 1, 65, 1, 1, 31, 7, 1, 1, 1, 31, 121, 1, 1, 7, 1, 1, 1, 5, 1, 13, 1, 7, 1, 1, 1, 7, 1, 57, 13

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OFFSET

1,4

COMMENTS

This is not multiplicative. The first point where a(m*n) = a(m)*a(n) does not hold for coprime m and n is 108 = 4*27, where a(108) = 70 <> 35 = 7*5 = a(4)*(27).

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..20000

Index entries for sequences related to sigma(n)

FORMULA

a(n) = A000203(n) / A348984(n) = sigma(n) / gcd(sigma(n), A325973(n)).

MATHEMATICA

f1[p_, e_] := p + 1; f2[p_, e_] := p^e + 1; s[1] = 1; s[n_] := (Times @@ f1 @@@ (f = FactorInteger[n]) + Times @@ f2 @@@ f)/2; a[n_] := Numerator[DivisorSigma[1, n]/s[n]]; Array[a, 100] (* Amiram Eldar, Nov 06 2021 *)

PROG

(PARI)

A325973(n) = (1/2)*sumdiv(n, d, d*(issquarefree(d) + (1==gcd(d, n/d))));

A348985(n) = { my(s=sigma(n)); (s/gcd(s, A325973(n))); };

CROSSREFS

Cf. A000203, A048250, A034448, A325973, A325974, A348984, A348986 (denominators).

Differs from A348048 for the first time at n=108, where a(108) = 70, while A348048(108) = 35.

Cf. also A348948.

Sequence in context: A366894 A088840 A348048 * A348504 A344696 A336457

Adjacent sequences: A348982 A348983 A348984 * A348986 A348987 A348988

KEYWORD

nonn,frac

AUTHOR

Antti Karttunen, Nov 06 2021

STATUS

approved