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Person# Anastasios Kyrillidis

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Related publications (23)

Related research domains (23)

Algorithm

In mathematics and computer science, an algorithm (ˈælɡərɪðəm) is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing calculations and data processing. More advanced algorithms can use conditionals to divert the code execution through various routes (referred to as automated decision-making) and deduce valid inferences (referred to as automated reasoning), achieving automation eventually.

Convex optimization

Convex optimization is a subfield of mathematical optimization that studies the problem of minimizing convex functions over convex sets (or, equivalently, maximizing concave functions over convex sets). Many classes of convex optimization problems admit polynomial-time algorithms, whereas mathematical optimization is in general NP-hard.

Computational complexity

In computer science, the computational complexity or simply complexity of an algorithm is the amount of resources required to run it. Particular focus is given to computation time (generally measured by the number of needed elementary operations) and memory storage requirements. The complexity of a problem is the complexity of the best algorithms that allow solving the problem. The study of the complexity of explicitly given algorithms is called analysis of algorithms, while the study of the complexity of problems is called computational complexity theory.

Volkan Cevher, Quoc Tran Dinh, Anastasios Kyrillidis

We propose a new proximal path-following framework for a class of constrained convex problems. We consider settings where the nonlinear-and possibly nonsmooth-objective part is endowed with a proximity operator, and the constraint set is equipped with a self-concordant barrier. Our approach relies on the following two main ideas. First, we reparameterize the optimality condition as an auxiliary problem, such that a good initial point is available; by doing so, a family of alternative paths toward the optimum is generated. Second, we combine the proximal operator with path-following ideas to design a single-phase, proximal path-following algorithm. We prove that our algorithm has the same worst-case iteration complexity bounds as in standard path-following methods from the literature but does not require an initial phase. Our framework also allows inexactness in the evaluation of proximal Newton directions, without sacrificing the worst-case iteration complexity. We demonstrate the merits of our algorithm via three numerical examples, where proximal operators play a key role.

Volkan Cevher, Anastasios Kyrillidis, Ya-Ping Hsieh, Rabeeh Karimi Mahabadi, Yu-Chun Kao

We study convex optimization problems that feature low-rank matrix solutions. In such scenarios, non-convex methods offer significant advantages over convex methods due to their lower space complexity as well as faster convergence speed. Moreover, many of these methods feature rigorous approximation guarantees. Non-convex algorithms are simple to analyze and implement as they perform Euclidean gradient descent on matrix factors. In contrast, this paper derives non-Euclidean optimization frame- work in the non-convex setting that takes nonlinear gradient steps on the factors. We prove convergence rates to the global minimum under appropriate assumptions. We provide numerical evidence with Fourier Ptychography and FastText applications using real data that shows our approach can significantly enhance solution quality

2018Volkan Cevher, Quoc Tran Dinh, Anastasios Kyrillidis

We propose a new proximal, path-following framework for a class of---possibly non-smooth---constrained convex problems. We consider settings where the non-smooth part is endowed with a proximity operator, and the constraint set is equipped with a self-concordant barrier. Our main contribution is a new re-parametrization of the optimality condition of the barrier problem, that allows us to process the objective function with its proximal operator within a new path following scheme. In particular, our approach relies on the following two main ideas. First, we re-parameterize the optimality condition as an auxiliary problem, such that a "good" initial point is available. Second, we combine the proximal operator of the objective and path-following ideas to design a single phase, proximal, path-following algorithm. Our method has several advantages. First, it allows handling non-smooth objectives via proximal operators, this avoids lifting the problem dimension via slack variables and additional constraints. Second, it consists of only a \emph{single phase} as compared to a two-phase algorithm in [43] In this work, we show how to overcome this difficulty in the proximal setting and prove that our scheme has the same O(ν√log(1/ε)) worst-case iteration-complexity with standard approaches [30, 33], but our method can handle nonsmooth objectives, where ν is the barrier parameter and ε is a desired accuracy. Finally, our framework allows errors in the calculation of proximal-Newton search directions, without sacrificing the worst-case iteration complexity. We demonstrate the merits of our algorithm via three numerical examples, where proximal operators play a key role to improve the performance over off-the-shelf interior-point solvers.