# A Convex Optimization Approach to Dynamic Programming in Continuous State and Action Spaces

## Abstract

In this paper, a convex optimization-based method is proposed for numerically solving dynamic programs in continuous state and action spaces. The key idea is to approximate the output of the Bellman operator at a particular state by the optimal value of a convex program. The approximate Bellman operator has a computational advantage because it involves a convex optimization problem in the case of control-affine systems and convex costs. Using this feature, we propose a simple dynamic programming algorithm to evaluate the approximate value function at pre-specified grid points by solving convex optimization problems in each iteration. We show that the proposed method approximates the optimal value function with a uniform convergence property in the case of convex optimal value functions. We also propose an interpolation-free design method for a control policy, of which performance converges uniformly to the optimum as the grid resolution becomes finer. When a nonlinear control-affine system is considered, the convex optimization approach provides an approximate policy with a provable suboptimality bound. For general cases, the proposed convex formulation of dynamic programming operators can be modified as a nonconvex bilevel program, in which the inner problem is a linear program, without losing the uniform convergence properties.

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1. However, our method is suitable for problems with high-dimensional action spaces.

2. More precisely, the set $${\mathcal {U}}({\varvec{x}})$$ needs to be represented by convex inequalities, i.e., there exist functions $$a_k: {\mathcal {X}} \times {\mathbb {R}}^m \rightarrow {\mathbb {R}}$$ and $$b_k: {\mathcal {X}} \rightarrow {\mathbb {R}}$$ such that

\begin{aligned} {\mathcal {U}}({\varvec{x}}) := \{ {\varvec{u}} \in {\mathbb {R}}^m : a_k ({\varvec{x}}, {\varvec{u}}) \le b_k({\varvec{x}}), k=1, \ldots , N_{ineq}\}, \end{aligned}

where $${\varvec{u}} \mapsto a_k ({\varvec{x}}, {\varvec{u}})$$ is a convex function for each fixed $${\varvec{x}} \in {\mathcal {X}}$$ and each k.

3. Note that the convexity of v is unused in the second part of the proof of Proposition 3.1. Thus, it is valid in the nonconvex case.

5. The CPU time increases superlinearly with the number of grid points. This is because the size of the optimization problem (5) also increases with the grid size. Note that the problem size is invariant when using the bi-level method in Sect. 4.2. Thus, in that case the CPU time scales linearly as shown in Table 3.

6. The observation of the second-order empirical convergence rate is consistent with our theoretical result since Theorem 3.1 only suggests that the suboptimality gap decreases with the first-order rate. Thus, the actual convergence rate can be higher than the convergence rate for the suboptimality gap.

7. To compute the optimal value function, we used the method in Sect. 4.2 discretizing the action space with 1001 equally spacing grid points.

8. The forward reachable set can be over-approximated in an analytical way, particularly when a loose approximation is allowed. For a high quality of approximation, one may use advanced computational techniques with semidefinite approximation  and ellipsoidal approximation , among others.

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## Acknowledgements

This work was supported in part by the Creative-Pioneering Researchers Program through SNU, the National Research Foundation of Korea funded by the MSIT (2020R1C1C1009766), and Samsung Electronics.

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Correspondence to Insoon Yang.

Communicated by Lars Grüne.

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## Appendix: State Space Discretization Using a Rectilinear Grid

### Appendix: State Space Discretization Using a Rectilinear Grid

In this appendix, we provide a concrete way to discretize the state space using a rectilinear grid. The construction below satisfies all the conditions in Sect. 2.3.

1. 1.

Choose a convex compact set $${\mathcal {Z}}_0 := [\underline{{\varvec{x}}}_{0, 1}, \overline{{\varvec{x}}}_{0, 1}] \times [\underline{{\varvec{x}}}_{0, 2}, \overline{{\varvec{x}}}_{0, 2}] \times \cdots \times [\underline{{\varvec{x}}}_{0, n}, \overline{{\varvec{x}}}_{0, n}]$$, and discretize it using an n-dimensional rectilinear grid. Set $$t \leftarrow 0$$.

2. 2.

Compute (or over-approximate) the forward reachable setFootnote 8

\begin{aligned} R_{t} := \big \{ f({\varvec{x}}, {\varvec{u}}, {\varvec{\xi }}) : {\varvec{x}} \in {\mathcal {Z}}_{t}, {\varvec{u}} \in {\mathcal {U}}({\varvec{x}}), {\varvec{\xi }} \in \varXi \big \}. \end{aligned}
3. 3.

Choose a convex compact set $${\mathcal {Z}}_{t+1} := [\underline{{\varvec{x}}}_{t+1, 1}, \overline{{\varvec{x}}}_{t+1, 1}] \times [\underline{{\varvec{x}}}_{t+1, 2}, \overline{{\varvec{x}}}_{t+1, 2}] \times \cdots \times [\underline{{\varvec{x}}}_{t+1, n}, \overline{{\varvec{x}}}_{t+1, n}]$$ such that $$R_t \subseteq {\mathcal {Z}}_{t+1}$$.

4. 4.

Expand the rectilinear grid to fit $${\mathcal {Z}}_{t+1}$$.

5. 5.

Stop if $$t+1 = K$$; otherwise, set $$t \leftarrow t+1$$ and go to Step 2.

We can then choose $${\mathcal {C}}_i$$ as each grid cell. We label $${\mathcal {C}}_i$$ so that $$\bigcup _{i=1}^{N_{{\mathcal {C}}, t}} {\mathcal {C}}_i = {\mathcal {Z}}_t$$ for all t. A two-dimensional example is shown in Fig. 1. This construction approach was used in Sects. 5.1 and 5.3.

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