Fuente TeX:

\sum_{n=0}^\infty a_{n+1}x^{n+1}=a_0a_0x+(a_0a_1+a_1a_0)x^2+(a_0a_2+a_1a_1+a_2a_0)x^3+...=x(a_0a_0+(a_0a_1+a_1a_0)x+(a_0a_2+a_1a_1+a_2a_0)x^2...)=x\sum_{n=0}^\infty c_nx^n=x(\sum_{n=0}^\infty a_nx^n\sum_{n=0}^\infty a_nx^n)=xf(x)f(x)