Delta cpq
From timescalewiki
Let $\mathbb{T}$ be a time scale and let $p$ and $q$ be rd-continuous functions that satisfy the relation $2p(t)+\mu(t)(p(t)^2+q(t)^2)=0$. The (alternative) cosine function is defined by $$c_{pq}(t,s) = \dfrac{e_{p+iq}(t,s)+e_{p-iq}(t,s)}{2},$$ where $e_{p+iq}$ denotes the delta exponential.
