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A280719 Expansion of (Sum_{k>=0} x^(k*(2*k-1)))^6. 2
1, 6, 15, 20, 15, 6, 7, 30, 60, 60, 30, 6, 15, 60, 90, 66, 45, 60, 80, 90, 66, 50, 120, 180, 135, 60, 15, 60, 186, 210, 141, 126, 120, 126, 165, 180, 241, 300, 210, 90, 90, 180, 270, 270, 210, 212, 270, 270, 200, 210, 366, 450, 390, 270, 135, 210, 375, 360, 396, 420, 300, 330, 375, 380, 510, 480, 336, 450, 510, 390, 330 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Number of ways to write n as an ordered sum of 6 hexagonal numbers (A000384).

a(n) > 0 for all n >= 0.

Every number is the sum of at most 6 hexagonal numbers.

Every number is the sum of at most k k-gonal numbers (Fermat's polygonal number theorem).

LINKS

Table of n, a(n) for n=0..70.

Ilya Gutkovskiy, Extended graphical example

Eric Weisstein's World of Mathematics, Hexagonal Number

Index to sequences related to polygonal numbers

FORMULA

G.f.: (Sum_{k>=0} x^(k*(2*k-1)))^6.

EXAMPLE

a(6) = 7 because we have:

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

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

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

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

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

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

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

MATHEMATICA

nmax = 70; CoefficientList[Series[Sum[x^(k (2 k - 1)), {k, 0, nmax}]^6, {x, 0, nmax}], x]

CROSSREFS

Cf. A000384, A007536, A008440, A045848, A280718, A282248.

Sequence in context: A063266 A131892 A291381 * A282173 A045848 A294651

Adjacent sequences:  A280716 A280717 A280718 * A280720 A280721 A280722

KEYWORD

nonn

AUTHOR

Ilya Gutkovskiy, Feb 10 2017

STATUS

approved

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Last modified February 20 14:53 EST 2018. Contains 299380 sequences. (Running on oeis4.)