# eigenvalue analysis vs fourier analysis for stability and their equivalence

I have a question regarding stability analysis for constant coefficient pde. Suppose I am looking at the pde $$u_t= au_{xx}$$ So the first approach is to compute the amplification factor, and this is so-called Fourier approach. Call it $Q(\zeta)$ and for explicit method plugging the test function $e^{ijhw}$ I have for $\zeta=hw$ that $$Q(\zeta)=1-4a\tau/h^2sin^2(\zeta/2)$$ On the other hand, the eigenvalue analysis for matrix $I+\tau A$, where $A$ is the discrete aproximation of $au_{xx}$ gives the eigenvalues of the matrix $I+\tau A$ that are exactly the same as the amplification factor $Q(\zeta)$.

Are they always the same, are these approaches equivalent? So, I can calculate either one and make sure it is bounded by $1$?

And second question. Since $A$ is symmetric, it implies $\max \lambda_i=||I+\tau A||_2$. However, if I use non uniform grid then $\max |\lambda_i| \leq ||I+\tau A||_2$ and thus neither of the two would guarantee stability because I can only show that $\max |\lambda_i|\leq 1$. What would be a sufficient condition for stability then in the case of a nonuniform grid?

If $A$ is a circulant matrix, then its eigenvectors are of the form $v_{j,w} = e^{ijhw}$; i.e., they are discrete Fourier modes. So Fourier analysis (or von Neumann analysis, as it's usually called) and eigenvalue analysis are equivalent if and only if you're dealing with a circulant matrix.
The maximum eigenvalue modulus and the Euclidean norm are equal if $A$ is a normal matrix. If $A$ is non-normal, then the Euclidean norm can be larger, as you say. A sufficient condition for stability is, of course, $\|I+\tau A\|_2 \le 1$. There are other sufficient conditions, depending on what kind of stability you mean. The most important kind of stability for PDE discretizations is Lax-Richtmeyer stability, which in your case would mean $$\|(I+\tau A)^n\|< C$$ for all $n\tau<T$, where $C$ is a constant that may depend on $T$ but not on $n$ or $\tau$. For a discussion, see e.g. Chapter 9 of Leveque's text.
Proving this kind of stability for a non-uniform grid discretization will probably be difficult. In practice, it is usually sufficient to consider the stability criterion based on a uniform grid, e.g. $\Delta t \le (\Delta x)^2/2$ and apply it but with $\Delta x$ replaced by the minimum of $\Delta x$ over the whole grid.