# American Institute of Mathematical Sciences

December  2019, 6(2): 171-198. doi: 10.3934/jcd.2019009

## Deep learning as optimal control problems: Models and numerical methods

 1 School of Mathematical Sciences, Queen Mary University of London, London E1 4NS, UK 2 Department of Mathematical Sciences, NTNU, 7491 Trondheim, Norway 3 Institute for Mathematical Innovation, University of Bath, Bath BA2 7JU, UK 4 Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK

* Corresponding author: Carola-Bibiane Schönlieb

Received  April 2019 Revised  September 2019 Published  November 2019

We consider recent work of [18] and [9], where deep learning neural networks have been interpreted as discretisations of an optimal control problem subject to an ordinary differential equation constraint. We review the first order conditions for optimality, and the conditions ensuring optimality after discretisation. This leads to a class of algorithms for solving the discrete optimal control problem which guarantee that the corresponding discrete necessary conditions for optimality are fulfilled. The differential equation setting lends itself to learning additional parameters such as the time discretisation. We explore this extension alongside natural constraints (e.g. time steps lie in a simplex). We compare these deep learning algorithms numerically in terms of induced flow and generalisation ability.

Citation: Martin Benning, Elena Celledoni, Matthias J. Ehrhardt, Brynjulf Owren, Carola-Bibiane Schönlieb. Deep learning as optimal control problems: Models and numerical methods. Journal of Computational Dynamics, 2019, 6 (2) : 171-198. doi: 10.3934/jcd.2019009
##### References:
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Bucy, Two-point boundary value problems of linear Hamiltonian systems, SIAM J. Appl. Math., 15 (1967), 1385-1389.  doi: 10.1137/0115121.  Google Scholar [7] M. Burger, G. Gilboa, S. Osher and J. Xu et al., Nonlinear inverse scale space methods, Commun. Math. Sci., 4 (2006), 179-212.  doi: 10.4310/CMS.2006.v4.n1.a7.  Google Scholar [8] M. Burger, S. Osher, J. Xu and G. Gilboa, Nonlinear inverse scale space methods for image restoration, in International Workshop on Variational, Geometric, and Level Set Methods in Computer Vision, Lecture Notes in Computer Science, 3752, Springer-Verlag, Berlin, 2005, 25–36. doi: 10.1007/11567646_3.  Google Scholar [9] B. Chang, L. Meng, E. Haber, L. Ruthotto, D. Begert and E. Holtham, Reversible architectures for arbitrarily deep residual neural networks, 32nd AAAI Conference on Artificial Intelligence, 2018. Google Scholar [10] T. Q. Chen, Y. Rubanova, J. Bettencourt and D. K. Duvenaud, Neural ordinary differential equations, in Advances in Neural Information Processing Systems, 2018, 6572–6583. Google Scholar [11] Y. N. Dauphin, R. Pascanu, C. Gulcehre, K. Cho, S. Ganguli and Y. Bengio, Identifying and attacking the saddle point problem in high-dimensional non-convex optimization, in Advances in Neural Information Processing Systems, 2014, 2933–2941. Google Scholar [12] A. L. Dontchev and W. W. Hager, The Euler approximation in state constrained optimal control, Math. Comp., 70 (2001), 173-203.  doi: 10.1090/S0025-5718-00-01184-4.  Google Scholar [13] J. Duchi, S. Shalev-Shwartz, Y. Singer and T. Chandra, Efficient projections onto the $\ell$1-ball for learning in high dimensions, in Proceedings of the 25th International Conference on Machine Learning, 2008, 272–279. Google Scholar [14] W. E, A proposal on machine learning via dynamical systems, Commun. Math. Stat., 5 (2017), 1-11.  doi: 10.1007/s40304-017-0103-z.  Google Scholar [15] F. Gay-Balmaz and T. S. Ratiu, Clebsch optimal control formulation in mechanics, J. Geom. Mech., 3 (2011), 41-79.  doi: 10.3934/jgm.2011.3.41.  Google Scholar [16] A. Gholami, K. Keutzer and G. Biros, ANODE: Unconditionally accurate memory-efficient gradients for neural ODEs, preprint, arXiv: math/1902.10298. Google Scholar [17] I. Goodfellow, J. Shlens and C. Szegedy, Explaining and harnessing adversarial examples, preprint, arXiv: math/1412.6572. Google Scholar [18] E. Haber and L. Ruthotto, Stable architectures for deep neural networks, Inverse Problems, 34 (2018), 22pp.  doi: 10.1088/1361-6420/aa9a90.  Google Scholar [19] W. W. Hager, Runge-Kutta methods in optimal control and the transformed adjoint system, Numer. Math., 87 (2000), 247-282.  doi: 10.1007/s002110000178.  Google Scholar [20] E. Hairer, C. Lubich and G. 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Igami, Artificial intelligence as structural estimation: Economic interpretations of Deep Blue, Bonanza, and AlphaGo, preprint, arXiv: math/1710.10967. Google Scholar [25] A. Kurakin, I. Goodfellow and S. Bengio, Adversarial examples in the physical world, preprint, arXiv: math/1607.02533. Google Scholar [26] W. Kutta, Beitrag zur näherungsweisen integration totaler differentialgleichungen, Z. Math. Phys., 46 (1901), 435-453.   Google Scholar [27] Y. LeCun, Y. Bengio and G. Hinton, Deep learning, Nature, 521 (2015), 436-444.  doi: 10.1038/nature14539.  Google Scholar [28] Y. LeCun, L. Bottou, Y. Bengio and P. Haffner, Gradient-based learning applied to document recognition, Proceedings of the IEEE, 86, 1998, 2278–2324. doi: 10.1109/5.726791.  Google Scholar [29] Y. LeCun, A theoretical framework for back-propagation, Proceedings of the 1988 Connectionist Models Summer School, 1, CMU, Pittsburgh, PA, 1988, 21–28. Google Scholar [30] Q. Li, L. Chen, C. Tai and E. Weinan, Maximum principle based algorithms for deep learning, J. Mach. Learn. Res., 18 (2017), 5998-6026.   Google Scholar [31] Q. Li and S. Hao, An optimal control approach to deep learning and applications to discrete-weight neural networks, preprint, arXiv: math/1803.01299. doi: 10.1109/TNNLS.2017.2665555.  Google Scholar [32] Y. Lu, A. Zhong, Q. Li and B. Dong, Beyond finite layer neural networks: Bridging deep architectures and numerical differential equations, in Proceedings of the 35th International Conference on Machine Learning, Proceedings of Machine Learning Research, 80, Stockholm, Sweden, 2018, 3276–3285. Available at: http://proceedings.mlr.press/v80/lu18d.html. Google Scholar [33] S.-M. Moosavi-Dezfooli, A. Fawzi and P. Frossard, Deepfool: A simple and accurate method to fool deep neural networks, in Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition, 2016, 2574–2582. doi: 10.1109/CVPR.2016.282.  Google Scholar [34] D. L. Phillips, A technique for the numerical solution of certain integral equations of the first kind, J. Assoc. Comput. Mach., 9 (1962), 84-97.  doi: 10.1145/321105.321114.  Google Scholar [35] D. C. Plaut et al., Experiments on learning by back propagation, work in progress. Google Scholar [36] L. Pontryagin et. al, Selected Works: The Mathematical Theory of Optimal Processes, Classics of Soviet Mathematics, 4, Gordon & Breach Science Publishers, New York, 1986.  Google Scholar [37] I. M. Ross, A roadmap for optimal control: The right way to commute, Ann. N.Y. Acad. Sci., 1065 (2005), 210-231.  doi: 10.1196/annals.1370.015.  Google Scholar [38] F. Santosa and W. W. Symes, Linear inversion of band-limited reflection seismograms, SIAM J. Sci. Statist. Comput., 7 (1986), 1307-1330.  doi: 10.1137/0907087.  Google Scholar [39] J. M. Sanz-Serna, Symplectic Runge-Kutta schemes for adjoint equations automatic differentiation, optimal control and more, SIAM Rev., 58 (2016), 3-33.  doi: 10.1137/151002769.  Google Scholar [40] O. Scherzer and C. Groetsch, Inverse scale space theory for inverse problems, in International Conference on Scale-Space Theories in Computer Vision, Lecture Notes in Computer Science, 2106, Springer, Berlin, 2001, 317–325. doi: 10.1007/3-540-47778-0_29.  Google Scholar [41] S. Sonoda and N. Murata, Double continuum limit of deep neural networks, ICML Workshop Principled Approaches to Deep Learning, 2017. Google Scholar [42] E. D. Sontag, Mathematical Control Theory: Deterministic Finite-Dimensional Systems, Texts in Applied Mathematics, 6, Springer-Verlag, New York, 1998. doi: 10.1007/978-1-4612-0577-7.  Google Scholar [43] I. Sutskever, J. Martens, G. E. Dahl and G. E. Hinton, On the importance of initialization and momentum in deep learning, Proceedings of the 30th International Conference on Machine Learning, 28, 2013, 1139–1147. Google Scholar [44] C. Szegedy, W. Zaremba, I. Sutskever, J. Bruna, D. Erhan, I. J. Goodfellow and R. Fergus, Intriguing properties of neural networks, International Conference on Learning Representations, 2014. Google Scholar [45] M. Thorpe and Y. van Gennip, Deep limits of residual neural networks, preprint, arXiv: math/1810.11741. Google Scholar [46] R. Tibshirani, Regression shrinkage and selection via the lasso, J. Roy. Statist. Soc. Ser. B, 58 (1996), 267-288.  doi: 10.1111/j.2517-6161.1996.tb02080.x.  Google Scholar [47] A. N. Tikhonov, Solution of incorrectly formulated problems and the regularization method, in Dokl. Akad. Nauk., 151, 1963, 1035–1038. Google Scholar [48] E. Weinan, J. Han and Q. Li, A mean-field optimal control formulation of deep learning, Res. Math. Sci., 6 (2019), 41pp.  doi: 10.1007/s40687-018-0172-y.  Google Scholar

show all references

##### References:
 [1] H. D. Abarbanel, P. J. Rozdeba and S. Shirman, Machine learning: Deepest learning as statistical data assimilation problems, Neural Comput., 30 (2018), 2025-2055.  doi: 10.1162/neco_a_01094.  Google Scholar [2] A. A. Agrachev and Y. Sachkov, Control Theory from the Geometric Viewpoint, Encyclopaedia of Mathematical Sciences, 87, Springer-Verlag, Berlin, 2004. doi: 10.1007/978-3-662-06404-7.  Google Scholar [3] M. Benning and M. Burger, Modern regularization methods for inverse problems, Acta Numer., 27 (2018), 1-111.  doi: 10.1017/S0962492918000016.  Google Scholar [4] C. M. Bishop, Pattern Recognition and Machine Learning, Information Science and Statistics, Springer, New York, 2006.  Google Scholar [5] S. Blanes and F. Casas, On the necessity of negative coefficients for operator splitting schemes of order higher than two, Appl. Numer. Math., 54 (2005), 23-37.  doi: 10.1016/j.apnum.2004.10.005.  Google Scholar [6] R. Bucy, Two-point boundary value problems of linear Hamiltonian systems, SIAM J. Appl. Math., 15 (1967), 1385-1389.  doi: 10.1137/0115121.  Google Scholar [7] M. Burger, G. Gilboa, S. Osher and J. Xu et al., Nonlinear inverse scale space methods, Commun. Math. Sci., 4 (2006), 179-212.  doi: 10.4310/CMS.2006.v4.n1.a7.  Google Scholar [8] M. Burger, S. Osher, J. Xu and G. Gilboa, Nonlinear inverse scale space methods for image restoration, in International Workshop on Variational, Geometric, and Level Set Methods in Computer Vision, Lecture Notes in Computer Science, 3752, Springer-Verlag, Berlin, 2005, 25–36. doi: 10.1007/11567646_3.  Google Scholar [9] B. Chang, L. Meng, E. Haber, L. Ruthotto, D. Begert and E. Holtham, Reversible architectures for arbitrarily deep residual neural networks, 32nd AAAI Conference on Artificial Intelligence, 2018. Google Scholar [10] T. Q. Chen, Y. Rubanova, J. Bettencourt and D. K. Duvenaud, Neural ordinary differential equations, in Advances in Neural Information Processing Systems, 2018, 6572–6583. Google Scholar [11] Y. N. Dauphin, R. Pascanu, C. Gulcehre, K. Cho, S. Ganguli and Y. Bengio, Identifying and attacking the saddle point problem in high-dimensional non-convex optimization, in Advances in Neural Information Processing Systems, 2014, 2933–2941. Google Scholar [12] A. L. Dontchev and W. W. Hager, The Euler approximation in state constrained optimal control, Math. Comp., 70 (2001), 173-203.  doi: 10.1090/S0025-5718-00-01184-4.  Google Scholar [13] J. Duchi, S. Shalev-Shwartz, Y. Singer and T. Chandra, Efficient projections onto the $\ell$1-ball for learning in high dimensions, in Proceedings of the 25th International Conference on Machine Learning, 2008, 272–279. Google Scholar [14] W. E, A proposal on machine learning via dynamical systems, Commun. Math. Stat., 5 (2017), 1-11.  doi: 10.1007/s40304-017-0103-z.  Google Scholar [15] F. Gay-Balmaz and T. S. Ratiu, Clebsch optimal control formulation in mechanics, J. Geom. Mech., 3 (2011), 41-79.  doi: 10.3934/jgm.2011.3.41.  Google Scholar [16] A. Gholami, K. Keutzer and G. Biros, ANODE: Unconditionally accurate memory-efficient gradients for neural ODEs, preprint, arXiv: math/1902.10298. Google Scholar [17] I. Goodfellow, J. Shlens and C. Szegedy, Explaining and harnessing adversarial examples, preprint, arXiv: math/1412.6572. Google Scholar [18] E. Haber and L. Ruthotto, Stable architectures for deep neural networks, Inverse Problems, 34 (2018), 22pp.  doi: 10.1088/1361-6420/aa9a90.  Google Scholar [19] W. W. Hager, Runge-Kutta methods in optimal control and the transformed adjoint system, Numer. Math., 87 (2000), 247-282.  doi: 10.1007/s002110000178.  Google Scholar [20] E. Hairer, C. Lubich and G. Wanner, Geometric Numerical Integration: Structure-Preserving Algorithms for Ordinary Differential Equations, Springer Series in Computational Mathematics, 31, Springer-Verlag, Berlin, 2006. doi: 10.1007/3-540-30666-8.  Google Scholar [21] K. He, X. Zhang, S. Ren and J. Sun, Identity mappings in deep residual networks, in European Conference on Computer Vision, Lecture Notes in Computer Science, 9908, Springer, Cham, 2016, 630–645. doi: 10.1007/978-3-319-46493-0_38.  Google Scholar [22] K. He, X. Zhang, S. Ren and J. Sun, Deep residual learning for image recognition, IEEE Conference on Computer Vision and Pattern Recognition, Las Vegas, NV, 2016, 770–778. doi: 10.1109/CVPR.2016.90.  Google Scholar [23] C. F. Higham and D. J. Higham, Deep learning: An introduction for applied mathematicians, preprint, arXiv: math/1801.05894. Google Scholar [24] M. Igami, Artificial intelligence as structural estimation: Economic interpretations of Deep Blue, Bonanza, and AlphaGo, preprint, arXiv: math/1710.10967. Google Scholar [25] A. Kurakin, I. Goodfellow and S. Bengio, Adversarial examples in the physical world, preprint, arXiv: math/1607.02533. Google Scholar [26] W. Kutta, Beitrag zur näherungsweisen integration totaler differentialgleichungen, Z. Math. Phys., 46 (1901), 435-453.   Google Scholar [27] Y. LeCun, Y. Bengio and G. Hinton, Deep learning, Nature, 521 (2015), 436-444.  doi: 10.1038/nature14539.  Google Scholar [28] Y. LeCun, L. Bottou, Y. Bengio and P. Haffner, Gradient-based learning applied to document recognition, Proceedings of the IEEE, 86, 1998, 2278–2324. doi: 10.1109/5.726791.  Google Scholar [29] Y. LeCun, A theoretical framework for back-propagation, Proceedings of the 1988 Connectionist Models Summer School, 1, CMU, Pittsburgh, PA, 1988, 21–28. Google Scholar [30] Q. Li, L. Chen, C. Tai and E. Weinan, Maximum principle based algorithms for deep learning, J. Mach. Learn. Res., 18 (2017), 5998-6026.   Google Scholar [31] Q. Li and S. Hao, An optimal control approach to deep learning and applications to discrete-weight neural networks, preprint, arXiv: math/1803.01299. doi: 10.1109/TNNLS.2017.2665555.  Google Scholar [32] Y. Lu, A. Zhong, Q. Li and B. Dong, Beyond finite layer neural networks: Bridging deep architectures and numerical differential equations, in Proceedings of the 35th International Conference on Machine Learning, Proceedings of Machine Learning Research, 80, Stockholm, Sweden, 2018, 3276–3285. Available at: http://proceedings.mlr.press/v80/lu18d.html. Google Scholar [33] S.-M. Moosavi-Dezfooli, A. Fawzi and P. Frossard, Deepfool: A simple and accurate method to fool deep neural networks, in Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition, 2016, 2574–2582. doi: 10.1109/CVPR.2016.282.  Google Scholar [34] D. L. Phillips, A technique for the numerical solution of certain integral equations of the first kind, J. Assoc. Comput. Mach., 9 (1962), 84-97.  doi: 10.1145/321105.321114.  Google Scholar [35] D. C. Plaut et al., Experiments on learning by back propagation, work in progress. Google Scholar [36] L. Pontryagin et. al, Selected Works: The Mathematical Theory of Optimal Processes, Classics of Soviet Mathematics, 4, Gordon & Breach Science Publishers, New York, 1986.  Google Scholar [37] I. M. Ross, A roadmap for optimal control: The right way to commute, Ann. N.Y. Acad. Sci., 1065 (2005), 210-231.  doi: 10.1196/annals.1370.015.  Google Scholar [38] F. Santosa and W. W. Symes, Linear inversion of band-limited reflection seismograms, SIAM J. Sci. Statist. Comput., 7 (1986), 1307-1330.  doi: 10.1137/0907087.  Google Scholar [39] J. M. Sanz-Serna, Symplectic Runge-Kutta schemes for adjoint equations automatic differentiation, optimal control and more, SIAM Rev., 58 (2016), 3-33.  doi: 10.1137/151002769.  Google Scholar [40] O. Scherzer and C. Groetsch, Inverse scale space theory for inverse problems, in International Conference on Scale-Space Theories in Computer Vision, Lecture Notes in Computer Science, 2106, Springer, Berlin, 2001, 317–325. doi: 10.1007/3-540-47778-0_29.  Google Scholar [41] S. Sonoda and N. Murata, Double continuum limit of deep neural networks, ICML Workshop Principled Approaches to Deep Learning, 2017. Google Scholar [42] E. D. Sontag, Mathematical Control Theory: Deterministic Finite-Dimensional Systems, Texts in Applied Mathematics, 6, Springer-Verlag, New York, 1998. doi: 10.1007/978-1-4612-0577-7.  Google Scholar [43] I. Sutskever, J. Martens, G. E. Dahl and G. E. Hinton, On the importance of initialization and momentum in deep learning, Proceedings of the 30th International Conference on Machine Learning, 28, 2013, 1139–1147. Google Scholar [44] C. Szegedy, W. Zaremba, I. Sutskever, J. Bruna, D. Erhan, I. J. Goodfellow and R. Fergus, Intriguing properties of neural networks, International Conference on Learning Representations, 2014. Google Scholar [45] M. Thorpe and Y. van Gennip, Deep limits of residual neural networks, preprint, arXiv: math/1810.11741. Google Scholar [46] R. Tibshirani, Regression shrinkage and selection via the lasso, J. Roy. Statist. Soc. Ser. B, 58 (1996), 267-288.  doi: 10.1111/j.2517-6161.1996.tb02080.x.  Google Scholar [47] A. N. Tikhonov, Solution of incorrectly formulated problems and the regularization method, in Dokl. Akad. Nauk., 151, 1963, 1035–1038. Google Scholar [48] E. Weinan, J. Han and Q. Li, A mean-field optimal control formulation of deep learning, Res. Math. Sci., 6 (2019), 41pp.  doi: 10.1007/s40687-018-0172-y.  Google Scholar
The four data sets used in the numerical study
Learned transformation and classifier for data set donut1d (top) and squares (bottom)
Snap shots of transformation of features for data set spiral
Learned transformation with fixed classifier for data set donut1d (top) and spiral (bottom)
Robustness on random initialisation for transformed data donut2d and linear classifier for two different initialisations
Function values over the course of the gradient descent iterations for data sets donut1d, donut2d, spiral, squares (left to right and top to bottom). The solid line represents training and the dashed line test data
Classification accuracy over the course of the gradient descent iterations for data setsdonut1d, donut2d, spiral, squares (left to right and top to bottom). The solid line represents training and the dashed line test data
Estimated time steps by ResNet/Euler, ODENet and ODENet+simplex for for data sets donut1d, donut2d, spiral, squares (left to right and top to bottom). ODENet+simplex consistently picks two to three time steps and set the rest to zero
Learned prediction and transformation for different Runge-Kutta methods and data sets spiral (top), donut2d (centre) and squares (bottom). All results are for 15 layers
Snap shots of the transition from initial to final state through the network with the data set spiral
Snap shots of the transition from initial to final state through the network with the data set donut2d
Snap shots of the transition from initial to final state through the network with the data set squares
Function values over the course of the gradient descent iterations for data sets donut1d, donut2d, spiral, squares (left to right and top to bottom)
Accuracy (left) and time steps (right) for MNIST100 dataset [28]
Features of testing examples from MNIST100 dataset [28] and transformed features by four networks under comparison: Net, ResNet, ODENet, ODENet+Simplex (from top to bottom). All networks have 20 layers
Four explicit Runge–Kutta methods: ResNet/Euler, Improved Euler, Kutta(3) and Kutta(4)
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