Thursday, February 4, 2016

A functional power series equation

I am interested in solving the following functional equation:


(1wzw)F(z,w)+zwnF(z,w2)=zzw.


Here n2 is a fixed integer and F(z,w) is a power series in two variables with complex coefficients, which you can assume that converges in a neighborhood of the origin in C2.


My thoughts: Defining G(j)=F(z,wj) for j1 and using the functional equation we obtain


G(j)zwjn1wjzwjajG(2j)=zzwj1wjzwjbj.


Dividing both sides by k=0a2kj=cj and defining H(j)=G(j)/cj we get


H(j)H(2j)=bjcj,


which implies


\begin{align*} F(z,w)=&\,G(1)=c_1H(1)\\ =&\,c_1\sum_{r=0}^\infty\bigl[H(2^r)-H(2^{r+1})\bigr]+c_1\lim_{r\to\infty}H(2^r)\\ =&\,\sum_{r=0}^\infty b_{2^r}\frac{c_1}{c_{2^r}}+\lim_{r\to\infty}\frac{c_1}{c_{2^r}}G(2^r)\\ %=&\,\sum_{r=0}^\infty \frac{z-zw^{2^r}}{1-w^{2^r}-zw^{2^r}}\,\prod_{k=0}^{r-1}\frac{zw^{2^kn}}{1-w^{2^k}-zw^{2^k}}+\lim_{r\to\infty}\frac{c_1}{c_{2^r}}G(2^r)\\ \end{align*}



Since F is holomorphic, then for |w|<1 we have lim by the functional equation. Defining


d_r=\frac{c_1}{c_{2^r}}=\prod_{k=0}^{r-1}\frac{-zw^{2^kn}}{1-w^{2^k}-zw^{2^k}}


we arrive to the following "explicit" formula, provided that the sequence \boldsymbol{(d_r)_{r\geq1}} converges for \boldsymbol{|z|,|w|} small:


F(z,w)=\sum_{r=0}^\infty b_{2^r}d_r+z\lim_{r\to\infty}d_r\,.


I personally believe that in fact (d_r)_{r\geq1} converges. Ironically, I wasn't lazy to think the partial solution above, but I am not feeling like solving the system of infinite linear equations satisfied by the coefficients of F. Who knows? some seasoned complex analyst can help me and obtain an explicit formula for both the series and the infinite product above.

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