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Geodesic and contour optimization using conformal mapping

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Abstract

We propose a novel optimization algorithm for differentiable functions utilizing geodesics and contours under conformal mapping. The algorithm can locate multiple optima by first following a geodesic curve to a local optimum then traveling to the next search area by following a contour curve. Alongside we implement a jumping mechanism which we call shadow casting to help geodesics jump to locations closer to the global optimum. To improve the efficiency, local search methods such as the Newton–Raphson algorithm are also employed. For functions with many optima or when the global optimum is very close to a local one, numerical analyses have shown that the resulting algorithm, SGEO-QN, can outperform recent derivative-free DIRECT variants in number of function/gradient evaluations. The results also indicate that under certain conditions, number of function/gradient evaluations for SGEO-QN scales nearly linearly with increasing dimensionality. Lastly, SGEO-QN appears to be less affected by rotational transforms of the objective functions than the variants of DIRECT compared.

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Notes

  1. We use a shorthand for partial derivatives, \(\partial _i \equiv \frac{\partial }{\partial x^i}\).

  2. Here, \(\mathbf {v}\) and \(\nabla f\) represent the tangent and gradient vectors in Euclidean space.

  3. Note that \( \sum _t f(\mathbf {x}_t) \ne 1\) and the vector \(\sum _t \widehat{f}(\mathbf {x}_t) (\mathbf {x}_t - \mathbf {A})\) will be approximately parallel to \((\mathbf {x}_t - \mathbf {A})\) in Eq. (6).

  4. Note that \(\mathbf {x}^*_{\gamma } \in L\).

  5. This is similar to the monotonic basin hopping algorithm [27] but we use a random walk instead of uniform sampling in a D-sphere.

  6. We implement quasi-Newton (QN) local searches on the geodesics \(\gamma \) and the line L. Hence we call the resulting algorithm SGEO-QN.

  7. A recent publication tackling the problem of comparing deterministic and stochastic optimization methods is discussed in [34].

  8. The only quantity that depends on the coordinate is \(t_C\) in Eq. (9), where a different choice of basis would result in different values in the components.

  9. The composition functions have the form \(f(x) = \sum _i \omega _i g_i(x)\), where i denotes the i-th basic function \(g_i(x)\) and its weight \(\omega _i \in [0,1]\), such that \(\sum _i \omega _i = 1\).

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Acknowledgments

We thank the anonymous reviewers for extremely useful feedbacks in assisting the writing of this manuscript.

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Correspondence to Ricky Fok.

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The authors declare that they have no conflict of interest.

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This research is supported by the Discovery Grants and Discovery Accelerator Supplement from Natural Sciences and Engineering Research Council of Canada (NSERC).

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Fok, R., An, A. & Wang, X. Geodesic and contour optimization using conformal mapping. J Glob Optim 69, 23–44 (2017). https://doi.org/10.1007/s10898-016-0467-8

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