A265712 Numbers n such that floor(Sum_{d|n} 1 / sigma(d)) = 2.

60, 72, 84, 90, 120, 144, 168, 180, 210, 216, 240, 252, 264, 270, 280, 288, 300, 312, 324, 330, 336, 360, 378, 384, 390, 396, 408, 420, 432, 450, 456, 462, 468, 480, 504, 510, 528, 540, 546, 552, 560, 570, 576, 588, 600, 612, 624, 630, 648, 660, 672, 684, 690

Offset

1,1

Comments

Numbers n such that A265710(n) = floor(A265708(n) / A069934(n)) = floor(A265709(n) / A265710(n)) = 2.

See A265714(n) = the smallest number k such that floor(Sum_{d|k} 1/sigma(d)) = n.

Links

G. C. Greubel, Table of n, a(n) for n = 1..7334

Example

60 is a term because floor(Sum_{d|60} 1/sigma(d)) = floor(155/72) = 2.

Mathematica

Select[Range@ 690, Floor[Sum[1/DivisorSigma[1, d], {d, Divisors@ #}]] == 2 &] (* Michael De Vlieger, Dec 31 2015 *)

Prog

(Magma) [n: n in [1..1000] | Floor(&+[1/SumOfDivisors(d): d in Divisors(n)]) eq 2]
(PARI) isok(n) = floor(sumdiv(n, d, 1/sigma(d))) == 2; \\ Michel Marcus, Dec 27 2015

Crossrefs

Cf. A069934, A000203, A265708, A265709, A265710, A265711, A265713, A265714, A266227, A266228.

Sequence in context: A036457 A030630 A068350 * A335198 A335935 A347935

Adjacent sequences: A265709 A265710 A265711 * A265713 A265714 A265715

Keyword

nonn

Author

Jaroslav Krizek, Dec 25 2015

Status

approved