PDEs Given a Parabolic PDE, one wishes to construct a weak solution by projecting a solution of the original PDE onto finite-dimensional subspaces of , so that the boundary/initial data is satisfied. The construction goes as follows:

  1. Take an orthonormal basis of by normalizing the eigenfunctions of (see Eigenvalues and Eigenfunctions of Elliptic Operators)
  2. Define the approximants: where the coefficients satisfy
  3. Solve the linear system of Ordinary Differential Equations: to find the unique Absolutely Continuous Function .
  4. Take and use energy estimates to show convergence to a unique weak solution: