Q-cos sub q
From specialfunctionswiki
The function $\cos_q$ is defined by $$\cos_q(z)=\dfrac{e_q(iz)+e_q(-iz)}{2}=\displaystyle\sum_{k=0}^{\infty} \dfrac{(-1)^kz^{2k}}{(q;q)_{2k}},$$ where $e_q$ denotes the $q$-exponential $e$ and $(q;q)_{2k}$ denotes the $q$-Pochhammer symbol.
