title: Lauricella's theorem
text: # Lauricella's theorem
  
  In the theory of orthogonal functions, Lauricella's theorem provides a condition for checking the closure of a set of orthogonal functions, namely:
  
  Theorem. A necessary and sufficient condition that a normal orthogonal set $\{u_{k}\}$ be closed is that the formal series for each function of a known closed normal orthogonal set $\{v_{k}\}$ in terms of $\{u_{k}\}$ converge in the mean to that function.
  
  The theorem was proved by Giuseppe Lauricella in 1912.
  
  