This thesis consists of three applications of Ranicki's algebraic theory of surgery to the topology of manifolds. The common theme is a decomposition of a global algebraic object into simple local pieces which models the decomposition of a global topological object into simple local pieces. Part I: Algebraic reconstruction of 4-manifolds. We extend the product and glueing constructions for symmetric Poincaré complexes, pairs and triads to a thickening construction for a symmetric Poincaré representation of a quiver. Gay and Kirby showed that, subject to certain conditions, the fold curves and fibres of a Morse 2-function F : M4 → Ʃ 2 determine a quiver of manifold and glueing data which allows one to reconstruct M and F up to diffeomorphism. The Gay-Kirby method of reconstructing M glues the pre-images of disc neighbourhoods of cusps and crossings with thickenings of regular fibres and thickenings of cobordisms between regular fibres. We use our thickening construction for a symmetric Poincaré representation of a quiver to give an algebraic analogue of the Gay-Kirby result to reconstruct the symmetric Poincaré complex (C(M); ϕ M) of M from a Morse 2-function. Part II: The L-theory of triangular matrix rings. We construct a chain duality on the category of left modules over a triangular matrix ring A = (A1;A2;B) where A1;A2 are rings with involution and B is an (A1;A2)-bimodule. We describe the resulting L-theory of A and relate it to the L-theory of A1;A2 and to the change of rings morphism B ⊗A2 − : A2-Mod → A1-Mod. By examining algebraic surgery over A we define a relative algebraic surgery operation on an (n+1)-dimensional symmetric Poincaré pair with data an (n+2)-dimensional triad. This gives an algebraic model for a half-surgery on a manifold with boundary. We then give an algebraic analogue of Borodzik, Némethi and Ranciki's half-handle decomposition of a relative manifold cobordism and show that every relative Poincaré cobordism is homotopy equivalent to a union of traces of elementary relative surgeries. Part III: Seifert matrices of braids with applications to isotopy and signatures. Let β be a braid with closure ^β a link. Collins developed an algorithm to find the Seifert matrix of the canonical Seifert surface Ʃ of ^ β constructed by Seifert's algorithm. Motivated by Collins' algorithm and a construction of Ghys, we define a 1-dimensional simplicial complex K(β) and a bilinear form λβ : C1(K(β);Z)×C1(K(β);Z) → Z[ 1/2 ] such that there is an inclusion K(β) ~ → Ʃ which is a homotopy equivalence inducing an isomorphism H1(Ʃ;Z) ≅ H1(K(β);Z) such that [λβ] : H1(K(β);Z) × H1(K(β);Z) → Z ⊂ Z[ 1/2 ] is the Seifert form of Ʃ. We show that this chain level model is additive under the concatenation of braids and then verify that this model is chain equivalent to Banchoff's combinatorial model for the linking number of two space polygons and Ranicki's surgery theoretic model for a chain level Seifert pairing. We then define the chain level Seifert pair (λβ; d β) of a braid β and equivalence relations, called A and Â-equivalence. Two n-strand braids are isotopic if and only if their chain level Seifert pairs are A-equivalent and this yields a universal representation of the braid group. Two n-strand braids have isotopic link closures in the solid torus D2 ×S1 if and only if their chain level Seifert pairs are  A-equivalent and this yields a representation of the braid group modulo conjugacy. We use the first representation to express the ω signature of a braid β in terms of the chain level Seifert pair (λ β; d β).

Let n > 1. We prove a homological stability theorem for the diffeomorphism groups of (4n+1)-dimensional manifolds, with respect to forming the connected sum with (2n-1)-connected, (4n+1)-dimensional manifolds that are stably parallelizable. Our techniques involve the study of the action of the diffeomorphism group of a manifold M on the linking form associated to the homology groups of M. In order to study this action we construct a geometric model for the linking form using the intersections of embedded and immersed Z/k-manifolds. In addition to our main homological stability theorem, we prove several results regarding disjunction for embeddings and immersions of Z/k-manifolds that could be of independent interest.