z 2 a 2 + 2 z 2 a 4 + z 2 a 6 + 2 a 4 − 1 a 8 {\displaystyle {\frac {z^{2}}{a^{2}}}+{\frac {2z^{2}}{a^{4}}}+{\frac {z^{2}}{a^{6}}}+{\frac {2}{a^{4}}}-{\frac {1}{a^{8}}}}
