The mass matrix and the stiffness matrix in finite element method for heat equation

1.4k Views Asked by At

On the page 99 in the Chapter 5.2 of Prof. Endre Süli's lecture notes on FEM for PDEs (see: https://people.maths.ox.ac.uk/suli/fem.pdf), he derived the mass matrix for forward Euler scheme and the stiffness matrix for backward Euler scheme for solving 1-d heat equation. And he said "It is a simple matter to show that this matrix is tri-diagonal and has the form ..." However, it seems not a simple matter to me. Could anyone read the page 98-99, and then provide me with a step-by-step illustration about how to derive the mass matrix and the stiffness matrix, and finally how to represent the evolution problem in a linear system like $\mathbf{A} \vec{U} = \vec{F}$?