− a 12 + 2 z 2 a 10 + a 10 − z 4 a 8 − z 4 a 6 − a 6 + 3 z 2 a 4 + 2 a 4 + z 2 a 2 {\displaystyle -a^{12}+2z^{2}a^{10}+a^{10}-z^{4}a^{8}-z^{4}a^{6}-a^{6}+3z^{2}a^{4}+2a^{4}+z^{2}a^{2}}