Linear Asymptotic Convergence of Anderson Acceleration: Fixed-Point Analysis

Abstract

We study the asymptotic convergence of AA($m$), i.e., Anderson acceleration with window size $m$ for accelerating fixed-point methods $x_{k+1}=q(x_{k})$, $x_k \in R^n$. Convergence acceleration by AA($m$) has been widely observed but is not well understood. We consider the case where the fixed-point iteration function $q(x)$ is differentiable and the convergence of the fixed-point method itself is root-linear. We identify numerically several conspicuous properties of AA($m$) convergence: First, AA($m$) sequences $\{x_k\}$ converge root-linearly but the root-linear convergence factor depends strongly on the initial condition. Second, the AA($m$) acceleration coefficients $β^{(k)}$ do not converge but oscillate as $\{x_k\}$ converges to $x^*$. To shed light on these observations, we write the AA($m$) iteration as an augmented fixed-point iteration $z_{k+1} =Ψ(z_k)$, $z_k \in R^{n(m+1)}$ and analyze the continuity and differentiability properties of $Ψ(z)$ and $β(z)$. We find that the vector of acceleration coefficients $β(z)$ is not continuous at the fixed point $z^*$. However, we show that, despite the discontinuity of $β(z)$, the iteration function $Ψ(z)$ is Lipschitz continuous and directionally differentiable at $z^*$ for AA(1), and we generalize this to AA($m$) with $m>1$ for most cases. Furthermore, we find that $Ψ(z)$ is not differentiable at $z^*$. We then discuss how these theoretical findings relate to the observed convergence behaviour of AA($m$). The discontinuity of $β(z)$ at $z^*$ allows $β^{(k)}$ to oscillate as $\{x_k\}$ converges to $x^*$, and the non-differentiability of $Ψ(z)$ allows AA($m$) sequences to converge with root-linear convergence factors that strongly depend on the initial condition. Additional numerical results illustrate our findings.