title: Perfect measure
text: # Perfect measure
  
  In mathematics — specifically, in measure theory — a perfect measure (or, more accurately, a perfect measure space) is one that is "well-behaved" in some sense. Intuitively, a perfect measure μ is one for which, if we consider the pushforward measure on the real line R, then every measurable set is "μ-approximately a Borel set". The notion of perfectness is closely related to tightness of measures: indeed, in metric spaces, tight measures are always perfect.
  
  ## Definition
  
  A measure space (X, Σ, μ) is said to be perfect if, for every Σ-measurable function f : X → R and every A ⊆ R with $f^{−1}(A)$ ∈ Σ, there exist Borel subsets $A_1$ and $A_2$ of R such that
  
      $A_{1}\subseteq A\subseteq A_{2}{\mbox{ and }}\mu {\bigf^{-1}(A_{2}\setminus A_{1}){\big )}=0.$
  
  ## Results concerning perfect measures
  
  * If X is any metric space and μ is an inner regular (or tight) measure on X, then (X, $B_X,$ μ) is a perfect measure space, where $B_X$ denotes the Borel σ-algebra on X.
  