title: P-curvature
text: # P-curvature
  
  In algebraic geometry, p-curvature is an invariant of a connection on a coherent sheaf for schemes of characteristic p > 0. It is a construction similar to a usual curvature, but only exists in finite characteristic.
  
  ## Definition
  
  Suppose X/S is a smooth morphism of schemes of finite characteristic p > 0, E a vector bundle on X, and $\nabla$ a connection on E. The p-curvature of $\nabla$ is a map $\psi :E\to E\otimes \Omega _{X/S}^{1}$ defined by
  
      $\psi (e)(D)=\nabla _{D}^{p}(e)-\nabla _{D^{p}}(e)$
  
  for any derivation D of ${\mathcal {O}}_{X}$ over S. Here we use that the pth power of a derivation is still a derivation over schemes of characteristic p.
  
  By the definition p-curvature measures the failure of the map $\operatorname {Der} _{X/S}\to \operatorname {End} (E)$ to be a homomorphism of restricted Lie algebras, just like the usual curvature in differential geometry measures how far this map is from being a homomorphism of Lie algebras.
  
  