We classify non-smooth del Pezzo surfaces with 1 3 (1, 1) points in 29 qG-deformation families grouped into six unprojection cascades (this overlaps with work of Fujita and Yasutake [14]), we tabulate their biregular invariants, we give good model constructions for surfaces in all families as degeneracy loci in rep quotient varieties, and we prove that precisely 26 families admit qG-degenerations to toric surfaces. This work is part of a program to study mirror symmetry for orbifold del Pezzo surfaces [2].In this paper we:(I) Classify non-smooth del Pezzo surfaces with 1 3 (1, 1) points in precisely 29 qG-deformation families. We further structure the classification into six unprojection cascades, determine their biregular invariants and their directed MMP together with a distinguished configuration of negative curves on the minimal resolution. This overlaps with work of Fujita and Yasutake [14].(II) Construct good models for surfaces in all families as degeneracy loci in rep quotient varieties. In all but two cases, the rep quotient variety is a simplicial toric variety.(III) Prove that precisely 26 of the 29 families admit a qG-degeneration to a toric surface.The classification is summarised in table 1 and table 2, which also plot invariants and provide good model constructions of surfaces in all families.This work is part of a program to understand mirror symmetry for orbifold del Pezzo surfaces [2,16,26,31,27,10] and it is aimed specifically at giving evidence for the conjectures made in [2].The rest of the introduction is organised as follows: in § 1.1 we give precise statements of our main results; in § 1.2 we say a few words about the context of [2]; in § 1.3 we outline the structure of the paper.
Our results1.1.1 The classification and its cascade structuren acts linearly on C 2 with weights a, b ∈ 1 n Z /Z. We always assume no stabilisers in codimension 0, 1, that is, hcf(a, n) = hcf(b, n) = 1. A del Pezzo surface is a surface X with cyclic quotient singularities and −K X ample. The Fano index of X is the largest positive integer f > 0 such that −K X = f A in the Class group Cl X.Remark 2. In this paper we view a del Pezzo surface X with quotient singularities as a variety. Such a surface is in a natural way a smooth orbifold (or DM stack), but we mostly ignore this structure. Thus for us Cl X is the Class group of Weil divisors on X modulo linear equivalence. In particular, although K X is a Cartier divisor on the orbifold, we think of it as a Q-Cartier divisor on the underlying variety (the coarse moduli space of the orbifold) and then to say that it is ample is to say that a positive integer multiple is Cartier and ample. See [2] for a discussion of qG-deformations of del Pezzo surfaces with cyclic quotient singularities. In particular, it is explained there that the singularity 1 3 (1, 1) is qG-rigid and the degree d = K 2 is locally constant in qG-families.We classify qG-deformation families of del Pezzo surfaces with k ≥ 1 1 3 (1, 1) points. It follows for example from the proof of [2, Lemma 6] tha...