A280210 - OEIS

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A280210

Expansion of (Sum_{k>=1} mu(k)^2*x^k)^3, where mu(k) is the Moebius function (A008683).

0, 0, 0, 1, 3, 6, 7, 9, 12, 19, 21, 21, 21, 30, 36, 37, 36, 48, 58, 63, 57, 70, 78, 87, 78, 96, 105, 114, 105, 123, 133, 138, 126, 148, 162, 174, 156, 195, 207, 220, 192, 234, 250, 261, 237, 280, 312, 318, 282, 330, 363, 370, 315, 375, 405, 432, 366, 421, 453, 483, 417, 468, 507, 532, 474, 537, 568, 591, 519, 601, 630, 666, 570

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,5

COMMENTS

Number of ordered ways of writing n as sum of three squarefree numbers (A005117).

LINKS

Table of n, a(n) for n=0..72.

Eric Weisstein's World of Mathematics, Squarefree

FORMULA

G.f.: (Sum_{k>=1} mu(k)^2*x^k)^3.

EXAMPLE

a(4) = 3 because we have [2, 1, 1], [1, 2, 1] and [1, 1, 2].