Space of initial conditions and geometry of two 4-dimensional discrete Painleve equations

A geometric study of two 4-dimensional mappings is given. By the resolution of indeterminacy they are lifted to pseudo-automorphisms of rational varieties obtained from $({mathbb P}^1)^4$ by blowing-up along sixteen 2-dimensional subvarieties. The symmetry groups, the invariants and the degree growth rates are computed from the linearisation on the corresponding Neron-Severi bilattices. It turns out that the deautonomised version of one of the mappings is a Backlund transformation of a direct product of the fourth Painleve equation which has $A_2^{(1)}+A_2^{(1)}$ type affine Weyl group symmetry, while that of the other mapping is of Noumi-Yamadas $A_5^{(1)}$ Painleve equation.