# Surgery obstruction groups

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## 1 Introduction

The surgery obstruction groups $L_n(\bZ\pi, w)$$\newcommand{\Z}{\mathbb{Z}} \newcommand{\R}{\mathbb{R}} \newcommand{\C}{\mathbb{C}} \newcommand{\N}{\mathbb{N}} \newcommand{\Q}{\mathbb{Q}} \newcommand{\F}{\mathbb{F}} \newcommand{\bZ}{\mathbb{Z}} \newcommand{\bR}{\mathbb{R}} \newcommand{\bC}{\mathbb{C}} \newcommand{\bH}{\mathbb{H}} \newcommand{\bQ}{\mathbb{Q}} \newcommand{\bF}{\mathbb{F}} \newcommand{\bN}{\mathbb{N}} \DeclareMathOperator\id{id} % identity map \DeclareMathOperator\Sq{Sq} % Steenrod squares \DeclareMathOperator\Homeo{Homeo} % group of homeomorphisms of a topoloical space \DeclareMathOperator\Diff{Diff} % group of diffeomorphisms of a smooth manifold \DeclareMathOperator\SDiff{SDiff} % diffeomorphism under some constraint \DeclareMathOperator\Hom{Hom} % homomrphism group \DeclareMathOperator\End{End} % endomorphism group \DeclareMathOperator\Aut{Aut} % automorphism group \DeclareMathOperator\Inn{Inn} % inner automorphisms \DeclareMathOperator\Out{Out} % outer automorphism group \DeclareMathOperator\vol{vol} % volume \newcommand{\GL}{\text{GL}} % general linear group \newcommand{\SL}{\text{SL}} % special linear group \newcommand{\SO}{\text{SO}} % special orthogonal group \newcommand{\O}{\text{O}} % orthogonal group \newcommand{\SU}{\text{SU}} % special unitary group \newcommand{\Spin}{\text{Spin}} % Spin group \newcommand{\RP}{\Rr\mathrm P} % real projective space \newcommand{\CP}{\Cc\mathrm P} % complex projective space \newcommand{\HP}{\Hh\mathrm P} % quaternionic projective space \newcommand{\Top}{\mathrm{Top}} % topological category \newcommand{\PL}{\mathrm{PL}} % piecewise linear category \newcommand{\Cat}{\mathrm{Cat}} % any category \newcommand{\KS}{\text{KS}} % Kirby-Siebenmann class \newcommand{\Hud}{\text{Hud}} % Hudson torus \newcommand{\Ker}{\text{Ker}} % Kernel \newcommand{\underbar}{\underline} %Classifying Spaces for Families of Subgroups \newcommand{\textup}{\text} \newcommand{\sp}{^}L_n(\bZ\pi, w)$ of C.T.C. Wall [Wall1999], [Wall1976] contain the obstructions to doing surgery on a degree 1 normal map $(f,b)\colon M \to X$$(f,b)\colon M \to X$ to obtain a homotopy equivalence. In this setting, $X$$X$ is an $n$$n$-dimensional PoincarĂ© complex $\pi = \pi_1(X, x_0)$$\pi = \pi_1(X, x_0)$ is the fundamental group of $X$$X$, and $w$$w$ is the orientation character of $X$$X$, which is determined by the first Stiefel-Whitney class of $X$$X$, $w_1(X)$$w_1(X)$.

A homotopy equivalence $f\colon M \to X$$f\colon M \to X$ has a Whitehead torsion $\tau(f) \in Wh(\pi)$$\tau(f) \in Wh(\pi)$ in the Whitehead group of $\pi$$\pi$. The surgery obstruction groups for surgery up to a homotopy equivalence with torsion in a prescribed subgroup $U \subseteq Wh(\pi)$$U \subseteq Wh(\pi)$ are denoted $L^U_n(\bZ, w)$$L^U_n(\bZ, w)$. The most important cases for geometric applications are $U = \{ 0 \}$$U = \{ 0 \}$, denoted $L^s_n(\bZ, w)$$L^s_n(\bZ, w)$, or $U = Wh(\pi)$$U = Wh(\pi)$, denoted $L^h_n(\bZ, w)$$L^h_n(\bZ, w)$. The main problems are the following:

1. to develop methods for the computation of the surgery obstruction groups,
2. to define invariants of degree 1 normal maps which detect the surgery obstruction.

The surgery obstruction groups $L_n(R\pi, w)$$L_n(R\pi, w)$ depend on a coefficient ring $R$$R$, a discrete group $\pi$$\pi$ and an orientation character $w\colon \pi \to \{\pm 1\}$$w\colon \pi \to \{\pm 1\}$. In general the surgery obstruction groups are abelian groups. For finite groups $\pi$$\pi$ the $L$$L$-groups are finitely-generated and the only torsion is $2$$2$-primary.