Polarized superspecial simple abelian surfaces with real Weil numbers

Abstract

Let q be an odd power of a prime p ∈ N, and PPSP( √ q ) be the finite set of isomorphism classes of principally polarized superspecial abelian surfaces in the simple isogeny class over Fq corresponding to the real Weil q-numbers ± √ q . We produce explicit formulas for PPSP( √ q ) of the following three kinds: (i) the class number formula, i.e. the cardinality of PPSP( √ q );(ii) the type number formula, i.e. the number of endomorphism rings up to isomorphism of members of PPSP( √ q ); (iii) the refined class number formula with respect to each finite group G, i.e. the number of elements of PPSP( √ q )whose automorphism group coincides with G. Similar formulas are obtained for other polarized superspecial members of this isogeny class using polarization modules. We observe several surprising identities involving the arithmetic genus of certain Hilbert modular surface on one side and the class number or type number of (P, P + )-polarized superspecial abelian surfaces in this isogeny class on the other side.