“…As described in the works of Wathen, Fried, and Lin, for the finite element and spectral element methods (and other methods for which the matrices are obtained by assembling elemental contributions), the eigenvalues λ m , for m = 1,…,n mn , of D are bounded by where and are the minimum and maximum eigenvalues of the elemental matrix D e : = ( M e ) −1 K e , respectively. This is known as the theorem of Irons and Treharne, and it implies that Therefore, a conservative choice for the time step that ensures stability is …”