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We propose a workshop focused on designing and implementing open-source code for studying the geometry and arithmetic of surfaces. We will emphasize the development of practical skills for computer experimentation. We have two key objectives: to implement cutting-edge algorithms for counting points and computing zeta functions for surfaces, and to develop functionality for manipulating indefinite lattices.

K3 surfaces form a natural testing ground for arithmetic and geometric conjectures. These surfaces bridge a dimensional gap: they may be viewed as higher-dimensional analogues of elliptic curves, whose arithmetic finds applications in number theory and cryptography, or as lower-dimensional versions of Calabi-Yau manifolds or holomorphic symplectic varieties. Historically, arithmetic geometry has focused on understanding the properties of curves. The recent development of algorithms for, e.g., zeta-function computations, provide an opportunity to make significant advances in computational understanding of surfaces.

A robust open-source software infrastructure is of central importance for testing and reproducing mathematical experiments. Without the ability to inspect and modify code, computational exploration is opaque. SageMath offers a widely used, accessible platform for creating and sharing research mathematical software.

Objectives. Our first objective is to incorporate new algorithms for counting points and computing zeta functions of K3 surfaces in SageMath. These algorithms allow us to test conjectures in arithmetic and explore the number-theoretic implications of mirror symmetry. Historically, the theoretical and computational framework for studying zeta functions of surfaces and higher-dimensional varieties has been very limited. This situation has changed in recent years, with pioneering work including that of van Luijk in [14], Elsenhans and Jahnel in [7] and Costa and Tschinkel in [5]. Meanwhile, theoretical descriptions of the way zeta functions for projective Calabi-Yau hypersurfaces vary under one-parameter deformations have been developed (cf. [8], [12], [15]). These developments have sparked renewed interest in the arithmetic implications of mirror symmetry for K3 surfaces and Calabi-Yau varieties more generally, as seen in [1] and [9]. Making code for computing zeta functions widely available will spur the development of new methods for analyzing the arithmetic of surfaces.

In computer algebra systems, methods for working with positive definite lattices are widely available. However, functionality for indefinite lattices is essentially non-existent. The second major objective of this workshop is to bridge this gap.

Indefinite integral lattices are omnipresent in algebraic geometry. The cup product on an algebraic surface equips its middle integral cohomology group (and thus its Neron-Severi group) with the structure of an indefinite lattice. In recent years, holomorphic symplectic manifolds have become a burgeoning area of research. Via the Bogomolov-Beauville-Fujiki quadratic form, we may give their second cohomology group a lattice structure as well.

Consider, in particular, a complex K3 surface. Its second integral, singular cohomology group is a lattice of signature (3,19) which admits a weight two Hodge structure (corresponding to a complex line of the complexified lattice). The Torelli Theorem for K3 surfaces states that this datum determines the surface up to isomorphism. Over the years, many people have employed the following strategy: pick an important geometric property of a K3 surface, reformulate the property in terms of lattices and Hodge structures, and apply the powerful techniques of lattice theory to prove a theorem. This strategy has been tremendously successful in the study of automorphisms and their fixed point sets, Brauer groups, holomorphic dynamics, moduli, elliptic fibrations and their Mordell-Weil lattices, and more recently in exploration of the Umbral Moonshine phenomenon.

Recently there has been some success in combining theoretical lattice-analysis techniques with computer aided calculations. See e.g. [2, 3, 4, 6, 10, 11, 13]. In each of these projects, the authors programmed their own implementation of the calculus of indefinite lattices. Having a reliable toolbox at hand would open the field to new participants and avoid constant re-invention of the wheel. Future applications of these techniques include generalizations to the holomorphic symplectic setting, where a Torelli theorem is now available, deepening connections to Umbral Moonshine and identifying missing automorphisms, and linking to the positive-characteristic setting and exploring the phenomenon wherein Picard ranks jump in a family of surfaces.

Western Canada in general and BIRS in particular is a natural center for advances in arithmetic geometry. Our workshop extends existing networks, building on the success of the Women in Numbers and Alberta Number Theory Days series. Simultaneously, our emphasis on developing concrete computational skills provides an excellent framework for engaging junior scholars and building research connections.