Ioannis Markakis

We study coinductive invertibility of cells in weak \(\omega\)\-categories. We use the inductive presentation of weak \(\omega\)\-categories via an adjunction with the category of computads, and show that invertible cells are closed under all operations of $\omega$\-categories. Moreover, we give a simple criterion for invertibility in computads, together with an algorithm computing the data witnessing the invertibility, including the inverse, and the cancellation data.

Two novel descriptions of weak \(\omega\)-categories have been recently proposed, using type-theoretic ideas. The first one is the dependent type theory CaTT whose models are \(\omega\)-categories. The second is a recursive description of a category of computads together with an adjunction to globular sets, such that the algebras for the induced monad are again \(\omega\)-categories. We compare the two descriptions by showing that there exits a fully faithful morphism of categories with families from the syntactic category of CaTT to the opposite of the category of computads, which gives an equivalence on the subcategory of finite computads. We derive a more direct connection between the category of models of CaTT and the category of algebras for the monad on globular sets, induced by the adjunction with computads.

We define inductively the opposites of a weak globular \(\omega\)-category with respect to a set of dimensions, and we show that the properties of being free on a globular set or a computad are preserved under forming opposites. We then provide a new description of hom \(\omega\)-categories, and show that the opposites of a hom \(\omega\)-category are hom \(\omega\)-categories of opposites of the original \(\omega\)-category.

We introduce a notion of signature whose sorts form a direct category, and study computads for such signatures. Algebras for such a signature are presheaves with an interpretation of every function symbol of the signature, and we describe how computads give rise to signatures. Generalising work of Batanin, we show that computads with certain generator-preserving morphisms form a presheaf category, and describe a forgetful functor from algebras to computads. Algebras free on a computad turn out to be the cofibrant objects for certain cofibrantly generated factorisation system, and the adjunction above induces the universal cofibrant replacement, in the sense of Garner, for this factorisation system. Finally, we conclude by explaining how many-sorted structures, weak \(\omega\)-categories, and algebraic semi-simplicial Kan complexes are algebras of such signatures, and we propose a notion of weak multiple category.

We give a new description of computads for weak globular ω-categories by giving an explicit inductive definition of the free words. This yields a new understanding of computads, and allows a new definition of \(\omega\)-category that avoids the technology of globular operads. Our framework permits direct proofs of important results via structural induction, and we use this to give new proofs that every ω-category is equivalent to a free one, and that the category of computads with variable-to-variable maps is a presheaf topos, giving a direct description of the index category. We prove that our resulting definition of \(\omega\)-category agrees with that of Batanin and Leinster and that the induced notion of cofibrant replacement for \(\omega\)-categories coincides with that of Garner.

Let \(X_P\) be the projective toric surface associated to a lattice polytope \(P\). If the number of lattice points lying on the boundary of \(P\) is at least \(4\), it is known that \(X_P\) is embeddable into a suitable projective space as zero set of finitely many quadrics. In this case, the determination of a minimal generating system of the toric ideal defining \(X_P\) is reduced to a simple Gaussian elimination.