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A245786 Numbers n such that k(n) = (n/tau(n) + sigma(n)/n) is an integer. 3
1, 672, 4680, 30240, 23569920, 45532800, 275890944, 14182439040, 153003540480, 403031236608, 518666803200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that A245784(n) / A245785(n) =  (n / A000005(n) + A000203(n) / n) is an integer.

Sequence of numbers k(n): 2, 31, 101, 319, 73660, 118579, …

Conjecture: Subsequence of A216793.

Refactorable multiply-perfect numbers (A245782) are members of this sequence.

a(12) > 10^13. - Giovanni Resta, Jul 13 2015

LINKS

Table of n, a(n) for n=1..11.

FORMULA

A245785(a(n)) = 1.

EXAMPLE

672 is in sequence because 672/tau(672) + sigma(672)/672 = 672/24 + 2016/672 = 31 (integer).

PROG

(MAGMA) [n: n in [1..100000] | (Denominator((n/(#[d: d in Divisors(n)])) + (SumOfDivisors(n)/n)) eq 1)]

(PARI) for(n=1, 10^8, s=n/numdiv(n); t=sigma(n)/n; if(floor(s+t)==s+t, print1(n, ", "))) \\ Derek Orr, Aug 15 2014

CROSSREFS

Cf. A000005, A000203, A245784, A245785, A245782.

Sequence in context: A057802 A231740 A245778 * A321116 A234481 A234476

Adjacent sequences:  A245783 A245784 A245785 * A245787 A245788 A245789

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Aug 15 2014

EXTENSIONS

a(7)-a(11) from Giovanni Resta, Jul 13 2015

STATUS

approved

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Last modified February 22 19:59 EST 2019. Contains 320403 sequences. (Running on oeis4.)