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Expansion of Sum_{k in Z} x^(2*k) / (1 - x^(7*k+3)).
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%I #17 Aug 01 2024 14:13:20

%S 1,0,0,1,1,0,1,-1,1,1,0,0,2,0,0,1,1,-1,0,0,1,2,0,0,2,0,0,0,1,-1,1,0,1,

%T 1,0,0,2,-1,1,2,0,0,0,0,1,1,0,-1,2,0,0,0,1,0,1,1,1,1,0,0,2,0,-1,1,1,0,

%U 0,-2,1,1,1,0,3,-1,0,2,1,-1,1,0,1,1,0,0,1,0,0,0,1,1,0,0,0,2,0,-1,2

%N Expansion of Sum_{k in Z} x^(2*k) / (1 - x^(7*k+3)).

%H R. P. Agarwal, <a href="https://www.ias.ac.in/describe/article/pmsc/103/03/0269-0293">Lambert series and Ramanujan</a>, Prod. Indian Acad. Sci. (Math. Sci.), v. 103, n. 3, 1993, pp. 269-293. see p. 286.

%F G.f.: Product_{k>0} (1-x^(7*k))^2 / ((1-x^(7*k-3)) * (1-x^(7*k-4))).

%F G.f.: Sum_{k in Z} x^(3*k) / (1 - x^(7*k+2)).

%o (PARI) my(N=100, x='x+O('x^N)); Vec(sum(k=-N, N, x^(2*k)/(1-x^(7*k+3))))

%o (PARI) my(N=100, x='x+O('x^N)); Vec(prod(k=1, N, (1-x^(7*k))^2/((1-x^(7*k-3))*(1-x^(7*k-4)))))

%Y Cf. A375106, A375108.

%K sign

%O 0,13

%A _Seiichi Manyama_, Jul 30 2024