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