  \zebra \koala \hen \eagle\bugs \beaver \swordfish \goat \wolf \frac { \partial ^ { { \kern 1 p t } 2 } \varrho _ { { \kern 1 p t } i } } { \partial { \kern 1 p t } t ^ 2 } - { \hat c } ^ { { \kern 1 p t } 2 } \triangle \varrho _ { { \kern 1 p t } i } = 0 ;