Sciences for the invitations and the excellent working conditions. The author would also like to thank Chris Rogers and Dmitry Roytenberg for their insightful remarks and Yunhe Sheng for his inspiration. The author is most grateful to Chenchang Zhu for many helpful discussions and suggestions over the years, and to Bruno Vallette for his collaboration, valuable insights, and hospitality on numerous occasions.
CHAPTER 1
PreliminariesThe aim of this chapter is to introduce notation and conventions used throughout this thesis.We denote by 𝐤 an arbitrary commutative unital ring and by 𝐤-Mod its category of modules. In practice, for any computations we will work over the integers 𝐤 = ℤ. Since ℤ is the initial object in the category of unital commutative rings, this ensures that our results hold over any such ring 𝐤.
Chain ComplexesThe index 𝑖 is referred to as (homological) degree, and we use the notation |𝑣| = 𝑖 for elements 𝑣 ∈ 𝑉 𝑖 . A morphism of chain complexes in 𝐤-modules 𝑓: (𝑉, d 𝑉 ) → (𝑊, d 𝑊 ) is a collection of 𝐤-linear maps {𝑓 𝑖 : 𝑉 𝑖 → 𝑊 𝑖 } 𝑖∈ℤ , such that d 𝑊 𝑖 ∘ 𝑓 𝑖 = 𝑓 𝑖−1 ∘ d 𝑉 𝑖 for all 𝑖 ∈ ℤ. The category of chain complexes in 𝐤-Mod is denoted by 𝐤-Ch. 1.1. Abelian structure. It is a standard result that 𝐤-Ch is an abelian category with biproduct the degreewise direct sum of 𝐤-modules (𝑉 ⊕ 𝑊) 𝑖 ≔ 𝑉 𝑖 ⊕ 𝑊 𝑖 equipped with the componentwise differential d 𝑉 ⊕𝑊 (𝑣 + 𝑤) = d 𝑉 (𝑣) + d 𝑊 (𝑤). Kernels and cokernels are computed degreewise, i.e., ker(𝑓) 𝑖 = ker(𝑓 𝑖 ) and coker(𝑓) 𝑖 = coker(𝑓 𝑖 ).
Symmetric monoidal structure.The category 𝐤-Ch can be equipped with a monoidal productSlightly abusing notation, the chain complex given by 𝐤 in degree 0 and 0 in all other degrees is denoted again by 𝐤. It acts as the unit object with respect to the monoidal product. The monoidal product satisfies a certain symmetry: we denote by 𝜏 the natural isomorphism with components given on homogeneous elementary tensors by (2) 𝜏 𝑉 ,𝑊 : 𝑉 ⊗ 𝑊 ⟶ 𝑊 ⊗ 𝑉 , 𝑣 ⊗ 𝑤 ⟼ (−1) |𝑣||𝑤| ⋅ 𝑤 ⊗ 𝑣 .Clearly 𝜏 𝑉 ,𝑊 ∘ 𝜏 𝑊 ,𝑉 = id, and together with the above monoidal structure this turns (𝐤-Ch, ⊗, 𝐤) into a symmetric monoidal category.Remark 1.2.1. The sign in the differential of the tensor product in Equation ( 1) is necessary to ensure that d 𝑉 ⊗𝑊 squares to zero, and, as a consequence, the sign in the symmetry isomorphism in Equation ( 2) is required such that the components of 𝜏 are 5 𝜒:𝐵→𝑚