Abstract
We present improvements to branch and bound techniques for globally optimizing functions with Lipschitz continuity properties by developing novel bounding procedures and parallelisation strategies. The bounding procedures involve nonconvex quadratic or cubic lower bounds on the objective and use estimates of the spectrum of the Hessian or derivative tensor, respectively. As the nonconvex lower bounds are only tractable if solved over Euclidean balls, we implement them in the context of a recent branch and bound algorithm (Fowkes et al. in J Glob Optim 56:1791–1815, 2013) that uses overlapping balls. Compared to the rectangular tessellations of traditional branch and bound, overlapping ball coverings result in an increased number of subproblems that need to be solved and hence makes the need for their parallelization even more stringent and challenging. We develop parallel variants based on both data- and task-parallel paradigms, which we test on an HPC cluster on standard test problems with promising results.
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Notes
Note that we need a larger set here as the balls in our overlapping covering extend outside the domain during the initial subdivisions.
Note that \(\fancyscript{B}\) does not need to be convex provided all line segments from \(x_{\fancyscript{B}}\) to \(x\) are contained in \(\fancyscript{B}\), i.e. if \(\fancyscript{B}\) is star-convex with star-centre \(x_{\fancyscript{B}}\).
Note that if \(\fancyscript{B}\) is not assumed to be compact, then (1.2) still holds provided the gradient \(g\) is Lipschitz continuous on the convex subdomain \(\fancyscript{B}\) and \(f \in C^1(\fancyscript{B})\).
Note that one can instead use the alternative definition of \(\ell ^2\)-eigenvalues that are the stationary points of the multilinear Rayleigh quotient for the \(\ell ^2\)-norm, \({Tx^3}/{||x ||_2^3}\) [28] and then the smallest \(\ell ^2\)-eigenvalue of \(T\), \(\lambda ^{\ell ^2}_{\min }(T)\) would be given by \(\lambda ^{\ell ^2}_{\min }(T) = \min _{x \ne 0} {Tx^3}/{||x ||_2^3}\). However, we will not use \(\ell ^2\)-eigenvalues here for reasons that will become clear later.
Note that an analogous result holds for \(\ell ^2\)-eigenvalues. Unfortunately, to the best of our knowledge, there are no known eigenvalue algorithms that are guaranteed to converge to the smallest \(\ell ^2\)-eigenvalue but it is possible to use generalisations of the power method using multiple starting points [22, 41]. However, this is not reliable as (1.3) requires a bound on the smallest eigenvalue and using multiple starting points does not guarantee this. Furthermore, there is no generalisation of Gershgorin’s Theorem for \(\ell ^2\)-eigenvalues, which is what we propose next for \(\ell ^3\)-eigenvalues.
Note that this bound appears in Section 6.2 of Fowkes et al. [16] but it is incorrect there.
Note that if \(\fancyscript{B}\) is not assumed to be compact, then (1.3) still holds provided the Hessian \(H\) is Lipschitz continuous on the convex subdomain \(\fancyscript{B}\) and \(f \in C^2(\fancyscript{B})\).
As correctly pointed out by an anonymous referee (and as used in Algorithm 3.2) one can optionally use \(\underline{f}(\fancyscript{B}) > U_k-\varepsilon \) as the condition for pruning which may allow the algorithm to discard more redundant balls.
Note that \(\fancyscript{R}\) is always a finite set. As the radius is halved each time a ball is split, there can only be a finite number of radii before numerical underflow occurs.
Note that since \(\fancyscript{L}^p\) is a priority queue w.r.t. \(\underline{f}(\fancyscript{B})\), \(\fancyscript{B}\) has the smallest lower bound \(\underline{f}(\fancyscript{B})\) of all balls in \(\fancyscript{L}^p\).
Note that another approach to test the efficiency of parallelism is to calculate the redundancy defined as \((F_P-F_1)/F_P\) when \(F_P>F_1\) and \(0\) otherwise, where \(F_1\) is the number of function evaluations of the serial algorithm and \(F_P\) the number of function evaluations of the parallel algorithm on \(P\) processors (see p. 324 of Strongin and Sergeyev [40]).
Note that the majority of problems in the COCONUT benchmark have nonlinear constraints that our algorithms cannot handle at present. This rather limited the number of problems we could actually test.
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The work of the first and second authors was supported by EPSRC grants EP/I028854/1 and NAIS EP/G036136/1 and the work of the third author by EP/I013067/1. We are also grateful to NAIS for funding computing time.
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Cartis, C., Fowkes, J.M. & Gould, N.I.M. Branching and bounding improvements for global optimization algorithms with Lipschitz continuity properties. J Glob Optim 61, 429–457 (2015). https://doi.org/10.1007/s10898-014-0199-6
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DOI: https://doi.org/10.1007/s10898-014-0199-6