Abstract
We study the convex hull of the mixed-integer set given by a conic quadratic inequality and indicator variables. Conic quadratic terms are often used to encode uncertainties, while the indicator variables are used to model fixed costs or enforce sparsity in the solutions. We provide the convex hull description of the set under consideration when the continuous variables are unbounded. We propose valid nonlinear inequalities for the bounded case, and show that they describe the convex hull for the two-variable case. All the proposed inequalities are described in the original space of variables, but extended SOCP-representable formulations are also given. We present computational experiments demonstrating the strength of the proposed formulations.
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Notes
If \(\varOmega =\nicefrac {-a(N)+b(N)}{\sqrt{\sum _{i\in N}c_i^2}}\), then the objective values corresponding to the solutions \(x=y=0\) and \(x=y=1\) have objective values of 0; in most cases, such solutions are optimal, resulting in initial/root gaps of infinity. Thus, we multiply \(\varOmega \) by 0.999 to ensure optimal solutions with objective value different from 0.
We also tested the cuts with data generated according to [7]. However in such cases the initial gaps are very small—less than 5%—and the simple linear cuts (17) result in close to 100% root gap improvement, thus the stronger inequalities yield a marginal improvement at best. In contrast, the instances generated according to [6] are more challenging, with initial gaps up to 80%.
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This paper is based upon work supported by the National Science Foundation under Grant No. 1818700.
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Gómez, A. Strong formulations for conic quadratic optimization with indicator variables. Math. Program. 188, 193–226 (2021). https://doi.org/10.1007/s10107-020-01508-y
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DOI: https://doi.org/10.1007/s10107-020-01508-y