A282349 - OEIS

A282349

Expansion of (Sum_{k>=0} x^(k*(2*k^2+1)/3))^7.

1, 7, 21, 35, 35, 21, 14, 43, 105, 140, 105, 42, 28, 105, 210, 210, 105, 21, 35, 147, 252, 245, 175, 105, 77, 154, 315, 455, 420, 210, 63, 147, 441, 630, 420, 105, 7, 147, 441, 525, 350, 210, 106, 126, 322, 567, 735, 560, 210, 84, 301, 840, 1050, 630, 210, 49, 315, 875, 980, 630, 245, 35, 245, 707, 1050, 980, 560, 210, 168

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OFFSET

0,2

COMMENTS

Number of ways to write n as an ordered sum of 7 octahedral numbers (A005900).

Pollock (1850) conjectured that every number is the sum of at most 7 octahedral numbers (a(n) > 0 for all n >= 0).

LINKS

Table of n, a(n) for n=0..68.

Ilya Gutkovskiy, Extended graphical example

Eric Weisstein's World of Mathematics, Octahedral Number

FORMULA

G.f.: (Sum_{k>=0} x^(k*(2*k^2+1)/3))^7.

EXAMPLE

a(6) = 14 because we have:

[6, 0, 0, 0, 0, 0, 0]

[0, 6, 0, 0, 0, 0, 0]

[0, 0, 6, 0, 0, 0, 0]

[0, 0, 0, 6, 0, 0, 0]

[0, 0, 0, 0, 6, 0, 0]

[0, 0, 0, 0, 0, 6, 0]

[0, 0, 0, 0, 0, 0, 6]

[1, 1, 1, 1, 1, 1, 0]

[1, 1, 1, 1, 1, 0, 1]

[1, 1, 1, 1, 0, 1, 1]

[1, 1, 1, 0, 1, 1, 1]

[1, 1, 0, 1, 1, 1, 1]

[1, 0, 1, 1, 1, 1, 1]

[0, 1, 1, 1, 1, 1, 1]

MATHEMATICA

nmax = 68; CoefficientList[Series[Sum[x^(k (2 k^2 + 1)/3), {k, 0, nmax}]^7, {x, 0, nmax}], x]

CROSSREFS

Cf. A005900, A282172.

Sequence in context: A173676 A131893 A282248 * A356454 A045849 A031008

Adjacent sequences: A282346 A282347 A282348 * A282350 A282351 A282352

KEYWORD

nonn

AUTHOR

Ilya Gutkovskiy, Feb 12 2017

STATUS

approved