Pomoc� pod�lov�ho krit�ria spo��t�me
$$
\lim _{n\rightarrow \infty }\left 
|{\frac {\left (n+1\right )^{3}z}{
{n}^{3}}}\right |1/3={\frac {
\left |z\right |}{3}}
$$
a vid�me �e $R=3$.
Na konvergen�n� kru�nici nen� spl�ena nutn� podm�nka 
konvergence ( $\abs . \abs = n^3 $]
