  In the paper, the authors prove that the generalized sine function
$\sin_{p,q}(x)$ and the generalized hyperbolic sine function $\sinh_{p,q}(x)$
are geometrically concave and geometrically convex, respectively. Consequently,
the authors verify a conjecture posed in the paper "B. A. Bhayo and M.
Vuorinen, On generalized trigonometric functions with two parameters, J.
Approx. Theory 164 (2012), no.~10, 1415\nobreakdash--1426; Available online at
\url{http://dx.doi.org/10.1016/j.jat.2012.06.003}".
