title: Fiber bundle construction theorem
text: # Fiber bundle construction theorem
  
  In mathematics, the fiber bundle construction theorem is a theorem which constructs a fiber bundle from a given base space, fiber and a suitable set of transition functions. The theorem also gives conditions under which two such bundles are isomorphic. The theorem is important in the associated bundle construction where one starts with a given bundle and surgically replaces the fiber with a new space while keeping all other data the same.
  
  ## Formal statement
  
  Let X and F be topological spaces and let G be a topological group with a continuous left action on F. Given an open cover ${U_i}$ of X and a set of continuous functions
  
      $t_{ij}:U_{i}\cap U_{j}\to G$
  
  defined on each nonempty overlap, such that the cocycle condition
  
      $t_{ik}(x)=t_{ij}(x)t_{jk}(x)\qquad \forall x\in U_{i}\cap U_{j}\cap U_{k}$
  
  holds, there exists a fiber bundle E → X with fiber F and structure group G that is trivializable over ${U_i}$ with transition functions $t_{ij}.$
  
  Let E′ be another fiber bundle with the same base space, fiber, structure group, and trivializing neighborhoods, but transition functions $t′_{ij}.$ If the action of G on F is faithful, then E′ and E are isomorphic if and only if there exist functions
  
      $t_{i}:U_{i}\to G$
  
  such that
  
      $t'_{ij}(x)=t_{i}(x)^{-1}t_{ij}(x)t_{j}(x)\qquad \forall x\in U_{i}\cap U_{j}.$
  
  Taking $t_i$ to be constant functions to the identity in G, we see that two fiber bundles with the same base, fiber, structure group, trivializing neighborhoods, and transition functions are isomorphic.
  
  A similar theorem holds in the smooth category, where X and Y are smooth manifolds, G is a Lie group with a smooth left action on Y and the maps $t_{ij}$ are all smooth.
  
  ## Construction
  
  The proof of the theorem is constructive. That is, it actually constructs a fiber bundle with the given properties. One starts by taking the disjoint union of the product spaces $U_i$ × F
  
      $T=\coprod _{i\in I}U_{i}\times F=\{(i,x,y):i\in I,x\in U_{i},y\in F\}$
  
  and then forms the quotient by the equivalence relation
  
      $(j,x,y)\sim (i,x,t_{ij}(x)\cdot y)\qquad \forall x\in U_{i}\cap U_{j},y\in F.$
  
  The total space E of the bundle is T/~ and the projection π : E → X is the map which sends the equivalence class of (i, x, y) to x. The local trivializations
  
      $\phi _{i}:\pi ^{-1}(U_{i})\to U_{i}\times F$
  
  are then defined by
  
      $\phi _{i}^{-1}(x,y)=[(i,x,y)].$
  
  ## Associated bundle
  
  Let E → X a fiber bundle with fiber F and structure group G, and let F′ be another left G-space. One can form an associated bundle E′ → X with a fiber F′ and structure group G by taking any local trivialization of E and replacing F by F′ in the construction theorem. If one takes F′ to be G with the action of left multiplication then one obtains the associated principal bundle.
  
  