“…Even in the second order equations, there are various discrete Painlevé equations which are not directly investigated in this paper, e.g., equations arising from the translations with different directions or length in the root lattice [55,109,121]. Also, we do not deal with higher order or multi-variable generalizations, which are now actively studied in relation with soliton equations [19,20,57,58,87,119,124,126], geometry of space of initial values [71,114], geometry of flag varieties [89], or general theory of monodromy preserving deformations [65]. The Painlevé equations are believed to define new transcendental functions, and it was rigorously proved for the Painlevé differential equations (see, for example [128,129]).…”