Finite-dimensional analogues of the string $s\leftrightarrow t$ duality and the pentagon equation

We use a variant of the functional pentagon equation (FPE) from the theory of integrable models as an algebraic explanation of the phenomenon known in physics as the $s\leftrightarrow t$ duality. We present two simple geometric examples of FPE solutions, one of which yields the Veneziano four-particle amplitude as a particular case. We interpret our FPE solutions in terms of relations in Lie groups.