

A205808


G.f.: Sum_{n=oo..oo} q^(9*n^2 + 2*n).


3



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OFFSET

0


LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000
S. Cooper and M. D. Hirschhorn, Results of Hurwitz type for three squares. Discrete Math. 274 (2004), no. 13, 924. See A(q).


FORMULA

Expansion of f(x^7, x^11) in powers of x where f(, ) is Ramanujan's general theta function.  Michael Somos, Jan 19 2017
Euler transform of a period 36 sequence.  Michael Somos, Jan 19 2017


EXAMPLE

G.f. = 1 + x^7 + x^11 + x^32 + x^40 + x^75 + x^87 + x^136 + x^152 + x^215 + ...
G.f. = q + q^64 + q^100 + q^289 + q^361 + q^676 + q^784 + q^1225 + q^1369 + ...


MATHEMATICA

a[ n_] := SeriesCoefficient[ QPochhammer[ x^7, x^18] QPochhammer[ x^11, x^18] QPochhammer[ x^18], {x, 0, n}]; (* Michael Somos, Jan 19 2017 *)


PROG

(PARI) {a(n) = issquare(9*n + 1)}; /* Michael Somos, Jan 19 2017 */


CROSSREFS

Characteristic function of A132355.
Sequence in context: A082784 A105165 A058342 * A238897 A297199 A185117
Adjacent sequences: A205805 A205806 A205807 * A205809 A205810 A205811


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Jan 31 2012


STATUS

approved



