Question: seriestoratpoly error

Question:seriestoratpoly error

Ex 85 Maple

opoly := (1-x^2)*diff(f(x), x$2) - x*diff(f(x), x) + n^2*f(x) = 0;
sol := op(2,dsolve(opoly));
gen := sum(cos(n*arcsin(x))*y^n/n!, n=0..7);

seriestoratpoly(series(gen, z, 8), ['egf']);

error not a series, why?

seriestoratpoly(gen, ['egf']);

refere to the following link, it is possible to use a few terms to calculate a generating function, but where is the function generating? how do it  work in gfun?


use infinity different from the result in

gen := sum(cos(n*arcsin(x))*y^n/n!, n=0..infinity);

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