# Convergence Rates of Inertial Forward-Backward Algorithms

Abstract : In a Hilbert space $\mathcal H$, assuming $(\alpha_k)$ a general sequence of nonnegative numbers, we analyze the convergence properties of the inertial forward-backward algorithm $(IFB)\{\begin{array}{l} y_k=x_k+\alpha_k(x_k-x_{k-1}), x_{k+1}={\rm prox}_{s\Psi}(y_k-s\nabla \Phi(y_k)) \end{array},$ where $\Psi: \mathcal H \to \mathbb R \cup \lbrace + \infty \rbrace$ is a proper lower-semicontinuous convex function, and $\Phi: \mathcal H \to \mathbb R$ is a differentiable convex function, whose gradient is Lipschitz continuous. Various options for the sequence $(\alpha_k)$ are considered in the literature. Among them, the Nesterov choice leads to the FISTA algorithm and accelerates convergence from $\mathcal{O}(1/k)$ to $\mathcal{O}(1/k^2)$ for the values. Several variants are used to guarantee the convergence of the iterates or to improve the rate of convergence for the values. For the design of fast optimization methods, the tuning of the sequence $(\alpha_k)$ is a subtle issue, which we deal with in this paper in general. We show that the convergence rate of the algorithm can be obtained simply by analyzing the sequence of positive real numbers $(\alpha_k)$. In addition to the case $\alpha_k= 1 -\frac{\alpha}{k}$ with $\alpha\geq 3$, our results apply equally well to $\alpha_k = 1- \frac{\alpha}{k^r}$, with an exponent \$0
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Journal articles

https://hal.archives-ouvertes.fr/hal-01962223
Contributor : Imb - Université de Bourgogne <>
Submitted on : Thursday, December 20, 2018 - 2:44:01 PM
Last modification on : Tuesday, May 28, 2019 - 1:54:04 PM

### Citation

Hedy Attouch, Alexandre Cabot. Convergence Rates of Inertial Forward-Backward Algorithms. SIAM Journal on Optimization, Society for Industrial and Applied Mathematics, 2018, 28 (1), pp.849-874. ⟨10.1137/17M1114739⟩. ⟨hal-01962223⟩

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