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