

A281228


Expansion of (Sum_{k>=0} x^(3^k))^2 [even terms only].


1



0, 1, 2, 1, 0, 2, 2, 0, 0, 1, 0, 0, 0, 0, 2, 2, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
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OFFSET

0,3


COMMENTS

Number of ways to write 2n as an ordered sum of two powers of 3.
First bisection of selfconvolution of characteristic function of powers of 3.


LINKS

Table of n, a(n) for n=0..105.
Index entries for sequences related to compositions


FORMULA

G.f.: (Sum_{k>=0} x^(3^k))^2 [even terms only].


EXAMPLE

G.f. = x^2 + 2*x^4 + x^6 + 2*x^10 + 2*x^12 + x^18 + 2*x^28 + 2*x^30 + 2*x^36 + ...
a(2) = 2 because we have [3, 1] and [1, 3].


MATHEMATICA

Take[CoefficientList[Series[Sum[x^3^k, {k, 0, 15}]^2, {x, 0, 260}], x], {1, 1, 2}]


CROSSREFS

Cf. A000244, A055235, A073267, A078932.
Sequence in context: A240808 A263142 A025253 * A284575 A112178 A134663
Adjacent sequences: A281225 A281226 A281227 * A281229 A281230 A281231


KEYWORD

nonn


AUTHOR

Ilya Gutkovskiy, Jan 18 2017


STATUS

approved



