Approximate dynamic programming for stochastic N-stage optimization with application to optimal consumption under uncertainty

Abstract

Stochastic optimization problems with an objective function that is additive over a finite number of stages are addressed. Although Dynamic Programming allows one to formally solve such problems, closed-form solutions can be derived only in particular cases. The search for suboptimal solutions via two approaches is addressed: approximation of the value functions and approximation of the optimal decision policies. The approximations take on the form of linear combinations of basis functions containing adjustable parameters to be optimized together with the coefficients of the combinations. Two kinds of basis functions are considered: Gaussians with varying centers and widths and sigmoids with varying weights and biases. The accuracies of such suboptimal solutions are investigated via estimates of the error propagation through the stages. Upper bounds are derived on the differences between the optimal value of the objective functional and its suboptimal values corresponding to the use at each stage of approximate value functions and approximate policies. Conditions under which the number of basis functions required for a desired approximation accuracy does not grow “too fast” with respect to the dimensions of the state and random vectors are provided. As an example of application, a multidimensional problem of optimal consumption under uncertainty is investigated, where consumers aim at maximizing a social utility function. Numerical simulations are provided, emphasizing computational pros and cons of the two approaches (i.e., value-function approximation and optimal-policy approximation) using the above-mentioned two kinds of basis functions. To investigate the dependencies of the performances on dimensionality, the numerical analysis is performed for various numbers of consumers. In the simulations, discretization techniques exploiting low-discrepancy sequences are used. Both theoretical and numerical results give insights into the possibility of co** with the curse of dimensionality in stochastic optimization problems whose decision strategies depend on large numbers of variables.

Appendix: Proofs and technical lemmas

Proof of Proposition 5

(i) We use a backward induction argument. For t=N−1,…,0, assume that at stage t+1, $\tilde{J}_{t+1} \in \mathcal {F}_{V,t+1}$ is such that $\| J_{t+1}-\tilde{J}_{t+1}\|_{{\mathrm{sup}}(\bar{Y}_{t+1})} \leq {\eta}_{t+1}$ for some η _t+1≥0. In particular, as $\tilde{J}_{N} = J_{N}$ one has η _N =0. Let $\hat{J}_{t}$ be defined as in formula (11). This can be written as $\hat{J}_{t}=T_{t} \tilde{J}_{t+1}$, where T _t is Bellman’s operator, defined for every bounded and continuous function k _t+1 on X _t+1×Z _t+1 as

$$(T_{t} k_{t+1}) (x_t,z_t) :=\sup_{y \in \mathcal{D}_t(x_t,z_t)} \Bigl[h_t(x_t, y,z_t) + \beta \mathop {{\mathbb{E}}}_{z_{t+1}|z_t} \bigl\{ k_{t+1}(y,z_{t+1})\bigr\} \Bigr]. $$

Thanks to Assumption 3, it follows by an application of [80, Theorem 9.6] that

$$\begin{aligned} \|J_{t}-\hat{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} =& \bigl\| (T_t J_{t+1})-(T_t \tilde{J}_{t+1})\bigr\| _{{\mathrm{sup}}(\bar{Y}_t)} \\ \leq &\beta \|J_{t+1}-\tilde{J}_{t+1}\|_{{\mathrm{sup}}(\bar{Y}_{t+1})} \leq \beta \eta_{t+1}. \end{aligned}$$

(51)

Before moving to the t-th stage, one has to find an approximation $\tilde{J}_{t} \in \mathcal {F}_{V,t}$ for J _t =T _t J _t+1. Such an approximation has to be obtained from $\hat{J}_{t}=T_{t} \tilde{J}_{t+1}$ (which, in general, may not belong to $\mathcal {F}_{V,t}$), because J _t =T _t J _t+1 is unknown.

For a bounded and continuous function f on $\bar{Y}_{t}$ and r>0, denote by B _r (f) the closed ball of radius r in the sup-norm over $\bar{Y}_{t}$ centered at f, i.e., $B_{r}(f):=\{g \ \mathrm{bounded} \ \mathrm{and}\ \mathrm{continuous}\ \mathrm{function}\ \mathrm{on}\ \bar{Y}_{t}\,|\, \|f-g\|_{{\mathrm{sup}}(\bar{Y}_{t})} \leq r \}$. Let $J_{t} \in B_{\beta\eta_{t+1}}(\hat{J}_{t})$. By assumption, there exists $f_{t} \in \mathcal {F}_{V,t}$ such that $\|J_{t}-f_{t}\|_{{\mathrm{sup}}(\bar{Y}_{t})} \leq \varepsilon_{t}$, i.e., $f_{t} \in B_{\varepsilon_{t}}(J_{t}) \cap \mathcal {F}_{V,t}$. Since J _t is unknown, one cannot simply set $\tilde{J}_{t}:=f_{t}$ but one can define⁷ $\tilde{J}_{t}$ as any element of the set

$$ \arg\min_{\tilde{f}_t \in \mathcal {F}_{V,t}} \|\hat{J}_t- \tilde{f}_t\|_{{\mathrm{sup}}(\bar{Y}_t)}. $$

(52)

Hence, by the assumption on f _t , the definition of $\tilde{J}_{t}$, and the triangle inequality, we get

$$\|\hat{J}_t-\tilde{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq \|\hat{J}_t-J_t\|_{{\mathrm{sup}}(\bar{Y}_t)} +\|J_t-f_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq \beta \eta_{t+1} + \varepsilon_t. $$

Moreover, by (51) we have

$$\|J_t-\tilde{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq \|J_t-\tilde{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} +\|\hat{J}_t-\tilde{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq 2 \beta \eta_{t+1} + \varepsilon_t :=\eta_t. $$

Starting from $\tilde{J}_{N} = J_{N}$ and making explicit the dependence of η _t from ε _t ,…,ε _N−1, after N−t iterations (t=0,…,N−1) we obtain

$$ \|J_t-\tilde{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq \eta_t= \varepsilon_t + 2\beta\, \varepsilon_{t+1} + \cdots+(2\beta)^{N-1-t}\,\varepsilon_{N-1}= \sum_{i=0}^{N-1-t}(2\beta)^i \varepsilon_{t+i}. $$

(53)

Then, we obtain $\|J-\tilde{J}\|_{{\mathrm{sup}}(\bar{Y}_{0})}=\|J_{0}-\tilde{J}_{0}\|_{{\mathrm{sup}}(\bar{Y}_{0})} \leq \sum_{i=0}^{N-1}{(2\beta)^{i}\varepsilon_{i}}$.

(ii) In order to prove (19), we need to find, for each t, suitable positive constants η _t , δ _t and χ _t such that

$$\begin{aligned} \|\tilde{J}_t-J_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq &\eta_t , \\ \|\tilde{J}_{V,t}-J_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq &\delta_t, \\ \|\tilde{J}_{V,t}-\tilde{J}_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq &\chi_t. \end{aligned}$$

In the following we show how one can define such constants in a recursive way and we prove that

$$\delta_0=2 \sum_{t=1}^{N-1} (2\beta)^t \varepsilon_t - 2 \sum_{t=1}^{N-1} \beta^t \varepsilon_t, $$

from which (19) follows. We divide this part of the proof in several steps.

Step 1: stage N−1.:

We recall that the policy $\tilde{g}_{V,N-1}(x_{N-1},z_{N-1})$ is defined as

$$\begin{aligned} & \tilde{g}_{V,N-1}(x_{N-1},z_{N-1}) \\ &\quad :=\arg\max_{y \in \mathcal{D}_{N-1}(x_{N-1},z_{N-1})} \Bigl[h_{N-1}(x_{N-1},y,z_{N-1}) + \beta \mathop {{\mathbb{E}}}_{z_N|z_{N-1}} \bigl\{ \tilde{J}_N (y,z_{N}) \bigr\} \Bigr]. \end{aligned}$$

(54)

Since $\tilde{J}_{N}=J_{N}$, the policy $\tilde{g}_{V,N-1}$ is optimal also for the problem (54) with $\tilde{J}_{N}$ replaced by J _N . Then we obtain $\tilde{J}_{V,N-1}=J_{N-1}$. Concluding, we can set δ _N−1:=0 and χ _N−1:=η _N−1.

Step 2: stage N−2.:

The reoptimized policy $\tilde{g}_{V,N-2}(x_{N-2},z_{N-2})$ is defined as

$$\begin{aligned} &\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}) \\ &\quad := \arg\max_{y \in \mathcal{D}_{N-2}(x_{N-2},z_{N-2})} \Bigl[h_{N-2}(x_{N-2},y,z_{N-2}) + \beta \mathop {{\mathbb{E}}}_{z_{N-1}|z_{N-2}} \bigl\{ \tilde{J}_{N-1} (y,z_{N-1}) \bigr\} \Bigr]. \end{aligned}$$

(55)

Now, we evaluate the distance of $\tilde{J}_{V,N-2}$ from J _N−2 in the sup-norm, then we determine δ _N−2. Since $\tilde{J}_{V,N-2}$ corresponds to the application of the suboptimal policies $\tilde{g}_{V,N-2}$ and $\tilde{g}_{V,N-1}$ at the stages N−2 and N−1, we get

$$ \tilde{J}_{V,N-2}(x_{N-2},z_{N-2}) \leq J_{N-2}(x_{N-2},z_{N-2}). $$

(56)

Moreover,

$$\begin{aligned} &\tilde{J}_{V,N-2}(x_{N-2},z_{N-2}) \\ &\quad =h_{N-2} \bigl(x_{N-2},\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \\ &\qquad {}+ \beta \mathop {{\mathbb{E}}}_{z_{N-1}|z_{N-2}} \bigl\{ \tilde{J}_{N-1} \bigl(\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \bigr\} \\ &\quad =h_{N-2}\bigl(x_{N-2},\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \\ &\qquad {}+ \beta \mathop {{\mathbb{E}}}_{z_{N-1}|z_{N-2}} \bigl\{ \tilde{J}_{N-1} \bigl( \tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \bigr\} \\ &\qquad {}+\beta \mathop {{\mathbb{E}}}_{z_{N-1}|z_{N-2}} \bigl\{ \tilde{J}_{V,N-1} \bigl( \tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \\ &\qquad {}-\tilde{J}_{N-1}\bigl(\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2} \bigr) \bigr\} \\ &\quad \geq J_{V,N-2}(x_{N-2},z_{N-2}) - \beta \eta_{N-1} - \beta \chi_{N-1}, \end{aligned}$$

(57)

where the last inequality in (57) is obtained observing that

$$\begin{aligned} &\bigl\vert \tilde{J}_{V,N-1} \bigl(\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \\ &\quad {}-\tilde{J}_{N-1}\bigl(\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2} \bigr)\bigr\vert \leq\chi_{N-1}, \\ & h_{N-2}\bigl(x_{N-2},\tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2} \bigr) \\ &\qquad {}+ \beta \mathop {{\mathbb{E}}}_{z_{N-1}|z_{N-2}} \bigl\{ \tilde{J}_{N-1} \bigl( \tilde{g}_{V,N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \bigr\} \\ &\quad =(T_{N-2} \tilde{J}_{N-1}) (x_{N-2},z_{N-2}), \\ & h_{N-2}\bigl(x_{N-2},g_{N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \\ &\qquad {}+\beta \mathop {{\mathbb{E}}}_{z_{N-1}|z_{N-2}} \bigl\{ J_{N-1}\bigl(g_{N-2}(x_{N-2},z_{N-2}),z_{N-2}\bigr) \bigr\} \\ &\quad =(T_{N-2} J_{N-1}) (x_{N-2},z_{N-2})= J_{N-2} (x_{N-2},z_{N-2}) \end{aligned}$$

(where T _N−2 is Bellman’s operator at the stage N−2), and

$$ \|T_{N-2} \tilde{J}_{N-1}-T_{N-2} J_{N-1}\|_{{\mathrm{sup}}(\bar{Y}_{N-2})} \leq \beta \|\tilde{J}_{N-1}-J_{N-1} \|_{{\mathrm{sup}}(\bar{Y}_{N-1})} \leq \beta \eta_{N-1}, $$

(58)

and the first inequality in (58) follows by an application of the contraction property of Bellman’s operator [80, Theorem 9.6].

Concluding, we obtain $\|\tilde{J}_{V,N-2}-J_{N-2}\|_{{\mathrm{sup}}(\bar{Y}_{N-2})} \leq \delta_{N-2}:= \beta(\eta_{N-1}+\chi_{N-1})$ and, by an application of the triangle inequality, $\|\tilde{J}_{V,N-2}-\tilde{J}_{N-2}\|_{{\mathrm{sup}}(\bar{Y}_{N-2})} \leq \chi_{N-2}:=\eta_{N-2}+\delta_{N-2}$.

Step 3: other stages.:

Similarly, we get the following recursive formulas for the other stages (t=N−3,…,0): $\|\tilde{J}_{V,t}-J_{t}\|_{{\mathrm{sup}}(\bar{Y}_{t})} \leq \delta_{t}:= \beta(\eta_{t+1}+\chi_{t+1})$ and $\|\tilde{J}_{V,t}-\tilde{J}_{t}\|_{{\mathrm{sup}}(\bar{Y}_{t})} \leq \chi_{t}:=\eta_{t}+\delta_{t}$.

Step 4: dependence of δ ₀ from the errors η ₀,…,η _N−1.:

$$ \begin{aligned} &\delta_{N-1}=0, \\ &\chi_{N-1}=\eta_{N-1}, \\ &\delta_{N-2}=\beta(\eta_{N-1}+\chi_{N-1})=2\beta \eta_{N-1}, \\ &\chi_{N-2}=\eta_{N-2}+\delta_{N-2}= \eta_{N-2}+2\beta \eta_{N-1}, \\ &\delta_{N-3}=\beta(\eta_{N-2}+\chi_{N-2})=2\beta \eta_{N-2} + 2 \beta^2 \eta_{N-1}, \\ &\chi_{N-3}=\eta_{N-3}+\delta_{N-3}= \eta_{N-3}+2\beta \eta_{N-2} + 2 \beta^2 \eta_{N-1}, \\ &\delta_{N-4}=\beta(\eta_{N-3}+\chi_{N-3})=2\beta \eta_{N-3} + 2 \beta^2 \eta_{N-2} + 2 \beta^3 \eta_{N-1}, \\ &\vdots \\ &\delta_{0}=2 \beta \eta_1+2 \beta^2 \eta_2 + 2 \beta^3 \eta_3 + \cdots + 2 \beta^{N-1} \eta_{N-1}=2 \sum_{i=1}^{N-1} \beta^i \eta_i. \end{aligned} $$

(59)

Step 5: dependence of δ ₀ from the errors ε ₁,…,ε _N−1.:

By (59) and the expressions (53) for η ₀,…,η _N−1, we make explicit the dependence of δ ₀ from the errors ε ₁,…,ε _N−1, as shown in the following:

$$\begin{aligned} \delta_{0} =& 2 \sum_{i=1}^{N-1} \sum_{j=0}^{N-1-i} \beta^i (2\beta)^j \varepsilon_{i+j} \\ =& 2 \bigl[\beta \varepsilon_1 + \bigl(\beta^2 + 2 \beta^2\bigr) \varepsilon_2 + \bigl(\beta^3 + 2 \beta^3 + 4 \beta^3\bigr) \varepsilon_3 + \cdots \\ &{}+ \bigl(\beta^{N-1} + 2 \beta^{N-1} + \cdots + 2^{N-2} \beta^{N-1}\bigr) \varepsilon_{N-1} \bigr] \\ =& 2 \sum_{i=1}^{N-1} \Biggl(\sum_{j=0}^{i-1} 2^j \Biggr) \beta^i \varepsilon_i = 2 \sum_{i=1}^{N-1} \bigl(2^i-1\bigr) \beta^i \varepsilon_i =2 \sum_{i=1}^{N-1} (2 \beta)^i \varepsilon_i - 2 \sum_{i=1}^{N-1} \beta^i \varepsilon_i, \end{aligned}$$

which concludes the proof.  □

Proof of Proposition 6

We compare the behavior of the two dynamical systems x _t+1=g _t (x _t ,z _t ) and $\tilde{x}_{t+1}=\tilde{g}_{P,t}(\tilde{x}_{t},z_{t})$, for the same initial state $\tilde{x}_{0}=x_{0}$ and the same sequence of random vectors z _t . By the Lipschitz continuity of h _t and h _N , we get

$$\begin{aligned} \|J -\tilde{J}_P\|_{{\mathrm{sup}}(\bar{Y}_0)} \leq &\mathop {{\mathbb{E}}}_{z_1,\ldots,z_N|z_0} \Biggl\{ \sum_{t=0}^{N-1} L_{h_t} \beta^t \sqrt{\|x_t-\tilde{x}_t \|^2 + \|x_{t+1}-\tilde{x}_{t+1}\|^2} \\ &{}+ \beta^N L_{h_N} \|x_N - \tilde{x}_N\| \Biggr\} . \end{aligned}$$

(60)

The Lipschitz continuity of g _t and the triangle inequality give

$$\begin{aligned} \|x_{t+1}-\tilde{x}_{t+1}\| =& \bigl\| g_t(x_{t},z_t)-\tilde{g}_{P,t}(\tilde{x}_t,z_t)\bigr\| \\ =& \bigl\| g_t(x_{t},z_t)-g_t( \tilde{x}_t,z_t) +g_t(\tilde{x}_t,z_t)-\tilde{g}_{P,t}(\tilde{x}_t,z_t)\bigr\| \\ \leq& \bigl\| g_t(x_{t},z_t)-g_t( \tilde{x}_t,z_t)\bigr\| + \bigl\| g_t(\tilde{x}_t,z_t)- \tilde{g}_{P,t}(\tilde{x}_t,z_t)\bigr\| \\ \leq& L_{g_t}\|x_t-\tilde{x}_t\| + \bigl\| g_t(\tilde{x}_t,z_t)-\tilde{g}_{P,t}(\tilde{x}_t,z_t)\bigr\| . \end{aligned}$$

(61)

Let $\tilde{\theta}_{t} :=\|x_{t}-\tilde{x}_{t}\|$. Then $\tilde{\theta}_{0}=0$ and Eq. (61) together with $\|g_{t} -\tilde{g}_{P,t}\|_{{\mathrm{sup}}(\bar{Y}_{t})} \leq {\varepsilon}_{t}$ provide $\tilde{\theta}_{t+1} \leq L_{g_{t}}\tilde{\theta}_{t}+{\varepsilon}_{t}$ for t=0,…,N−1. So, the upper bound $\|J {-}\,\tilde{J}_{P}\|_{{\mathrm{sup}}(\bar{Y}_{0})}\leq \sum_{t=0}^{N-1} \beta^{t} L_{h_{t}} \sqrt{\theta_{t}^{2}+\theta_{t+1}^{2}}+\beta^{N} L_{h_{N}}\theta_{N}$ is obtained by (60) and the definition of θ _t (which does not depend on the particular sequence of random vectors z _t ). As θ _t is nonnegative for t=0,…,N−1, we have $\sqrt{\theta_{t}^{2}+\theta_{t+1}^{2}} \leq \theta_{t} +\theta_{t+1}$. This, together with the upper bound $\|J -\tilde{J}_{P}\|_{{\mathrm{sup}}(\bar{Y}_{0})}\leq \sum_{t=0}^{N-1} \beta^{t} L_{h_{t}} \sqrt{\theta_{t}^{2}+\theta_{t+1}^{2}}+\beta^{N} L_{h_{N}}\theta_{N}$ and the linear difference equation $\theta_{t+1}=L_{g_{t}}\theta_{t}+{\varepsilon}_{t}$, yields the estimate $\|J -\tilde{J}_{P}\|_{{\mathrm{sup}}(\bar{Y}_{0})} \leq \sum_{t=0}^{N-1} \varGamma_{t} {\varepsilon}_{t}$.  □

The next two lemmas provide smoothness properties of value functions and optimal policies. Lemma 17 (which gives high-order continuity properties) will be used in the proof of Lemma 18, which, in turn, will be exploited in the proofs of Propositions 7 and 8.

Lemma 17

Under Assumption 2, $J_{t} \in \mathcal {C}^{m}(X_{t} \times Z_{t})$, t=0,…,N, and the optimal policy g _t satisfies $g_{t} \in \mathcal {C}^{m-1}(X_{t} \times Z_{t}, \mathbb {R}^{d})$, t=0,…,N−1.

Proof of Lemma 17

The regularity conditions stated in Assumption 2 (among these, Assumption 2(iii), which is used only in this part of the proof) guarantee that solving Problem $\varSigma_{N}^{\mathrm{stoch}}$ is equivalent to solving iteratively Bellman’s equations (this follows, e.g., by an application of [48, Theorem 2.6]). The remaining of the proof works by backward induction as described in the following.

Suppose that, for t=0,…,N−1 and the value of α _t+1>0 given in Assumption 2(iv) for t=N−1 and (v) for t=0,…,N−2, one knows that $J_{t+1}(x_{t+1},z_{t+1}) \in \mathcal {C}^{m}(X_{t+1} \times Z_{t+1})$ and is α _t+1-concave in x _t+1 (i.e., if one fixes z _t+1, then J _t+1(x _t+1,z _t+1) is α _t+1-concave as a function of x _t+1). For t=N−1 this is guaranteed by Assumption 2(iv). We show that also $J_{t}(x_{t},z_{t}) \in \mathcal {C}^{m}(X_{t} \times Z_{t})$ is α _t -concave in its first argument (i.e., the function J _t (⋅,z _t ) is α _t -concave for every fixed z _t ∈Z _t ), with the same value of α _t >0 given in Assumption 2(v).

Let $(x_{t}, z_{t}) \in \operatorname {int}(X_{t} \times Z_{t})$ and $J^{e}_{t+1}(y,z_{t}):=\mathop {{\mathbb{E}}}_{z_{t+1}|z_{t}} \{J_{t+1}(y,z_{t+1}) \}$. By computing the conditional expected value, it is easy to see that $J^{e}_{t+1}(y,z_{t})$ is α _t+1-concave in y. As $J_{t+1} \in \mathcal {C}^{m}(X_{t+1} \times Z_{t+1})$ and $p_{z_{t+1}|{z_{t}}} \in \mathcal {C}^{m}(Z_{t+1} \times Z_{t})$, by interchanging derivatives and conditional expectations we get $J^{e}_{t+1} \in \mathcal {C}^{m}(X_{t+1} \times Z_{t})$. Since by hypothesis the optimal policy g _t is interior, the first-order optimality condition

$$ \nabla_2 h_t\bigl(x_t,g_t(x_t,z_t),z_t\bigr)+\beta \nabla_1 J^e_{t+1} \bigl(g_t(x_t,z_t),z_t\bigr)=0 $$

(62)

holds. Then, by the Implicit Function Theorem we get

$$\begin{aligned} \nabla_{x_t} g_t(x_t,z_t) = &-\bigl[\nabla^2_{2,2} \bigl(h_t\bigl(x_t, g_t(x_t,z_t),z_t \bigr) \bigr)+ \beta \nabla^2_{1,1} J^e_{t+1} \bigl(g_t(x_t,z_t),z_t\bigr)\bigr]^{-1} \\ &{}\cdot\nabla^2_{2,1}h_t\bigl(x_t,g_t(x_t,z_t),z_t\bigr) \end{aligned}$$

(63)

and

$$\begin{aligned} \nabla_{z_t} g_t(x_t,z_t) =& -\bigl[\nabla^2_{2,2} \bigl(h_t\bigl(x_t,g_t(x_t,z_t),z_t\bigr) \bigr) + \beta \nabla^2_{1,1} J^e_{t+1}\bigl(g_t(x_t,z_t),z_t\bigr)\bigr]^{-1} \\ &{}\cdot \nabla^2_{2,3}h_t\bigl(x_t,g_t(x_t,z_t),z_t \bigr), \end{aligned}$$

(64)

where the matrix $\nabla^{2}_{2,2} (h_{t}(x_{t},g_{t}(x_{t},z_{t}),z_{t}) )+ \beta \nabla^{2} J^{e}_{t+1}(g_{t}(x_{t},z_{t}))$ is non-singular. Indeed, by the concavity of h _t with respect to its second argument (which holds by Assumption 2(v)), the matrix $\nabla^{2}_{2,2} (h_{t}(x_{t},g_{t}(x_{t},z_{t}),z_{t}))$ is negative semi-definite, and the matrix $\nabla^{2}_{1,1} J^{e}_{t+1}(g_{t}(x_{t},z_{t}),z_{t})$ is negative definite since $J^{e}_{t+1}$ is α _t+1-concave in the first argument and α _t+1>0. Hence, the matrix $\nabla^{2}_{2,2} (h_{t}(x_{t},g_{t}(x_{t},z_{t}),z_{t}) )+ \beta \nabla^{2} J^{e}_{t+1}(g_{t}(x_{t},z_{t}))$ is non-singular and so (63) and (64) are well defined.

By differentiating (63) and (64) up to the order m, we get $g_{t} \in C^{m-1}(\operatorname {int}(X_{t} \times Z_{t}), \mathbb {R}^{d})$. As the expressions obtained for its partial derivatives up to the order m−1 are bounded and continuous not only on $\operatorname {int}(X_{t} \times Z_{t})$ but on the whole X _t ×Z _t , one has $g_{t} \in C^{m-1}(X_{t} \times Z_{t}, \mathbb {R}^{d})$.

Since $J_{t}(x_{t},z_{t})=h_{t}(x_{t},g_{t}(x_{t}),z_{t})+\beta J^{e}_{t+1}(g_{t}(x_{t},z_{t}),z_{t})$, by differentiating (62) we get

$$\begin{aligned} \bigl(\nabla_{x_t} J_t(x_t,z_t) \bigr)^{T} =& \bigl(\nabla_1 h_t \bigl(x_t,g_t(x_t,z_t),z_t\bigr)\bigr)^{T} + \bigl[\nabla_2 h_t\bigl(x_t,g_t(x_t,z_t),z_t\bigr) \\ &{}+ \beta \nabla_1 J^e_{t+1} \bigl(g_t(x_t,z_t),z_t\bigr)\bigr]^{T} \nabla_{x_t} g_t(x_t,z_t) \\ =& \bigl(\nabla_1 h_t\bigl(x_t,g_t(x_t,z_t),z_t \bigr)\bigr)^{T}, \end{aligned}$$

(65)

where the last equality holds by the first-order optimality condition (62). Similarly, we have

$$\begin{aligned} \bigl(\nabla_{z_t} J_t(x_t,z_t) \bigr)^{T} =& \bigl(\nabla_3 h_t \bigl(x_t,g_t(x_t,z_t),z_t \bigr)\bigr)^{T} +\bigl[\nabla_2 h_t \bigl(x_t,g_t(x_t,z_t),z_t\bigr) \\ &{}+ \beta \nabla_1 J^e_{t+1} \bigl(g_t(x_t,z_t),z_t\bigr)\bigr]^{T} \nabla_{z_t} g_t(x_t,z_t) \\ &{}+ \beta \bigl(\nabla_2 J^e_{t+1} \bigl(g_t(x_t,z_t),z_t\bigr)\bigr)^{T} \\ =&\bigl(\nabla_1 h_t\bigl(x_t,g_t(x_t,z_t),z_t \bigr)\bigr)^{T} + \beta \bigl(\nabla_2 J^e_{t+1}\bigl(g_t(x_t,z_t),z_t \bigr)\bigr)^{T}\!. \end{aligned}$$

(66)

By differentiating (65) and (66) up to the order m, we get $J_{t} \in \mathcal {C}^{m}(\operatorname {int}(X_{t} \times Z_{t}))$. Likewise for the optimal policies, this extends to $J_{t} \in \mathcal {C}^{m}(X_{t} \times Z_{t})$.

In order to conclude the backward induction step, it remains to show that J _t is α _t -concave in its first argument for the same value of α _t >0 in Assumption 2(v). By [58, Proposition 1], for every $a_{0} \in \mathbb {R}^{d}$ and z _t fixed we get

$$\begin{aligned} a_0^{T}\nabla^2_{1,1} J_t(x_t,z_t)a_0 =&\max _{a_1 \in \mathbb {R}^d} \bigl[(a_0,a_1)^{T} \nabla^2_{(1,2)} h_t\bigl(x_t,g_t(x_t,z_t),z_t \bigr) (a_0,a_1) \\ &{}+ \beta a_1^{T}\nabla^2_{1,1} J^e_{t+1}\bigl(g_t(x_t,z_t),z_t \bigr)a_1 \bigr]. \end{aligned}$$

(67)

By Assumption 2(v) and the concavity of $J^{e}_{t+1}$ with respect to the first argument, it follows that $a_{0}^{T}\nabla^{2}_{1,1} J_{t}(x_{t},z_{t})a_{0} \leq -\alpha_{t} \|a_{0}\|^{2}$. As a twice continuously differentiable function is α-concave iff the maximum eigenvalue of its Hessian is at most −α, it follows that J _t is α _t -concave in its first argument.  □

Recall that for r>0 and a positive integer l, the Bessel potential of order r on $\mathbb {R}^{l}$, denoted by G _r (x), is the inverse Fourier transform of the function ${\hat {G}_{r}}(\omega):= (2\pi)^{-\frac{d}{2}}(1+\|{\omega}\|^{2})^{-r/2}$. It is continuous when r>l [79, p. 132]. For r>0 and 1≤p≤∞, $\mathcal {B}^{r}_{p}(\mathbb {R}^{l})$ is the Bessel potential space, whose elements are functions u such that u=f∗G _r , where $f \in \mathcal {L}_{p}(\mathbb {R}^{l})$ [79, p. 134].

For an open set $\varOmega \subseteq \mathbb {R}^{l}$, a positive integer m, and 1≤p≤∞, by $\mathcal {W}_{p}^{m}(\varOmega)$ we denote the Sobolev space of functions whose partial derivatives up to the order m are in $\mathcal {L}_{p}(\varOmega)$.

Lemma 18

Let Assumption 2 be satisfied. Then the following hold.

• Case 1: m≥2 and 1<p<∞.

  • (i) For every t=0,…,N there exists a function $\bar{J}^{p}_{t} \in \mathcal {B}^{m}_{p}(\mathbb {R}^{d+s})$ such that $J_{t}=\bar{J}^{p}_{t}|_{X_{t} \times Z_{t}}$.

  • (ii) For every t=0,…,N−1 and j=1,…,d there exists a function $\bar{g}_{t,j}^{p} \in \mathcal {B}^{m-1}_{p}(\mathbb {R}^{d+s})$ such that, for the component g _t,j of the optimal policy g _t , one has $g_{t,j}=\bar{g}^{p}_{t,j}|_{X_{t} \times Z_{t}}$.

• Case 2: m≥2 and p=1.

  • (iii) If m=2, then (i) holds with $\mathcal {B}^{m}_{p}(\mathbb {R}^{d+s})$ replaced by $\mathcal {B}^{2}_{1}(\mathbb {R}^{d+s})$.

  • (iv) If m>2 is even, then (i) holds with $\mathcal {B}^{m}_{p}(\mathbb {R}^{d+s})$ replaced by $\mathcal {B}^{m}_{1}(\mathbb {R}^{d+s})$ and (ii) holds with $\mathcal {B}^{m-1}_{p}(\mathbb {R}^{d+s})$ replaced by $\mathcal {B}^{m-2}_{1}(\mathbb {R}^{d+s})$.

  • (v) If m is odd, then (i) holds with $\mathcal {B}^{m}_{p}(\mathbb {R}^{d+s})$ replaced by $\mathcal {B}^{m-1}_{1}(\mathbb {R}^{d+s})$ and (ii) holds with $\mathcal {B}^{m-1}_{p}(\mathbb {R}^{d+s})$ replaced by $\mathcal {B}^{m-1}_{1}(\mathbb {R}^{d+s})$.

Proof of Lemma 18

• (i) By Lemma 17 and the compactness of X _t ×Z _t , we get $J_{t} \in \mathcal {C}^{m}(X_{t} \times Z_{t}) \subset \mathcal {W}^{m}_{p}(\operatorname {int}(X_{t} \times Z_{t}))$. As X _t is bounded and convex, by [38, Corollary 1.2.2.3, p. 12] it has a Lipschitz boundary. By Assumption 2(i), (ii) X _t ×Z _t has a Lipschitz boundary, too. So, by Sobolev’s extension theorem [79, Theorem 5, p. 181], which holds thanks to the regularity of the boundary, J _t can be extended for every 1≤p≤∞ on the whole $\mathbb {R}^{d+s}$ to a function $\bar{J}_{t}^{p} \in \mathcal {W}^{m}_{p}(\mathbb {R}^{d+s})$. By the equivalence between Sobolev spaces and Bessel potential spaces [79, Theorem 3, p. 135], which holds for 1<p<∞, we have $\bar{J}^{p} \in \mathcal {B}^{m}_{p}(\mathbb {R}^{d+s})$ which, when restricted to X _t ×Z _t , proves (i).

• (ii) By Lemma 17, $g_{t,j} \in \mathcal {C}^{m-1}(X_{t} \times Z_{t}) \subset \mathcal {W}^{m-1}_{p}(\operatorname {int}(X_{t} \times Z_{t}))$. Then the statement follows by similar arguments as in the last part of the proof of (i).

• (iii), (iv), and (v) As d+s>1, for l even the inclusion $\mathcal {W}^{l}_{1}(\mathbb {R}^{d+s}) \subset \mathcal {B}^{l}_{1}(\mathbb {R}^{d+s})$ holds [79, Remark 6.6(b), p. 160]. Then (iii), (iv), and (v) follow by combining this with $J_{t} \in \mathcal {W}^{m}_{1}(\mathbb {R}^{d+s})$ and $g_{t,j} \in \mathcal {W}^{m-1}_{1}(\mathbb {R}^{d+s})$.  □

The next lemma, which is proven in [34, Sect. 5] and will be used in the proof of Proposition 7, states an approximation property that holds for functions belonging to the Bessel potential space $\mathcal {B}^{r}_{1}(\mathbb {R}^{l})$, when r>l.

Lemma 19

[34, Corollary 5.2]

Let l be a positive integer and r>l. Then for every $f \in \mathcal {B}^{r}_{1}(\mathbb {R}^{l})$ and every positive integer n there exist $t_{1}, \ldots, t_{n} \in \mathbb {R}^{l}$, b ₁,…,b _n >0, δ ₁,…,δ _n ∈{−1,+1}, and an absolute constant C>0 such that

$$\sup_{x \in \mathbb {R}^l} \Biggl| f({x})-\frac{K(r,l)}{n} \sum_{i=1}^n \delta_i\, e^{-\frac{\|x-t_i\|^2}{b_i}}\Biggr| \leq C\,K(r,l) \sqrt {\frac{l+3}{n}}, $$

where $K(r,l):= 2^{-l/2}\,\frac{\varGamma(r/2-d/2)}{\varGamma(r/2)}\|\lambda\|_{\mathcal {L}_{1}(\mathbb {R}^{l})}$ and $\varGamma(z) := \int_{0}^{\infty}y^{z-1}e^{-y}\,dy$ is the Gamma function.

Proof of Proposition 7

• (i) Let d+s be even. By Lemma 18, there exists a function $\bar{J}^{o,1}_{t} \in \mathcal {B}^{d+s+2}_{1}(\mathbb {R}^{d+s})$ such that $J_{t}=\bar{J}^{o,1}_{t}|_{X_{t} \times Z_{t}}$. Since the order r=d+s+2 of the Bessel potential associated to $\mathcal {B}^{r}_{1}(\mathbb {R}^{d+s})$ is larger than the number of variables d+s, we conclude the proof by Lemma 19.

  For d+s odd, by Lemma 18 there exists a function $\bar{J}_{t} \in \mathcal {B}^{d+s+1}_{1}(\mathbb {R}^{d+s})$ such that $J^{o,1}_{t}=\bar{J}^{o,1}_{t}|_{X_{t} \times Z_{t}}$. As the order r=d+s+1 of the Bessel potential associated to $\mathcal {B}^{r}_{1}(\mathbb {R}^{d+s})$ is larger than the number of variables d+s, we conclude the proof by Lemma 19.

• (ii) It is proven like (i) by replacing m, J _t , and $\bar{J}^{o,1}_{t}$ with m−1, g _t , and $\bar{g}^{o,1}_{t}$, resp.  □

The next lemma, which is an immediate consequence of [56, Theorem 2.1], will be used in the proof of Proposition 8. Recall that in Sect. 4 we have defined

$$\begin{aligned} \mathcal {S}^\prime :=\biggl\{ &\psi:\mathbb {R}\to \mathbb {R}| {\rm nonzero, \ infinitely \ many \ times \ differentiable \ in \ some}\ \mathcal {I}\subset \mathbb {R}, \\ &{\rm open \ interval \ and \ such \ that \ there \ exists} \ c \in \mathcal {I}: \frac{\partial^k \psi}{\partial z^k} \biggm|_{z=c} \neq 0\ \forall k \in \mathbb {N}\biggr\} . \end{aligned}$$

Lemma 20

Let r and l be positive integers and $\psi \in \mathcal {S}^{\prime}$. Then for every $f \in \mathcal {C}^{r}(\mathbb {R}^{l})$ and every positive integer n there exist $a_{1}, \ldots, a_{n} \in \mathbb {R}^{l}$, $b_{1}, \ldots, b_{n} \in \mathbb {R}$, $\delta_{1}, \ldots, \delta_{n} \in \mathbb {R}$, and a constant C>0 (we emphasize that C does not depend on f) such that

$$ \sup_{x \in \mathbb {R}^l} \Biggl| f({x})-\sum _{i=1}^n \delta_i \psi\bigl(\langle a_i , x \rangle + b_i\bigr)\Biggr| \leq C n^{-{r}/{l}}. $$

(68)

Proof of Proposition 8

One proceeds as the proof of Proposition 7, using Lemma 20 instead of Lemma 18 and noting that for r≥⌊l/2⌋+1, the second member of (68) can be replaced by Cn ^−1/2.  □

Proof of Theorem 9

One combines Proposition 7(i) with Proposition 5(i) and (ii). □

Proof of Theorem 10

Proceeding as in the proof of Lemma 17, we obtain that the t-th value function J _t and the t-th optimal policy g _t are Lipschitz continuous. Hence, for t=0,…,N−1 and any sequence of ε _t >0, Proposition 6(ii) gives

$$ \|J-\tilde{J}_{P, \mathit{GRBFN}}\|_{{\mathrm{sup}}(\bar{Y}_0)} \leq \sum_{t=0}^{N-1} \varGamma_t {\varepsilon}_t $$

(69)

for any sequence of functions $\gamma_{t} \in \mathcal {G}(n_{t,1},\ldots,n_{t,d}; \bar{Y}_{t}, X_{t+1})$ such that $\|g_{t} -\gamma_{t}\|_{{\mathrm{sup}}(\bar{Y}_{t})} \leq {\varepsilon}_{t}$ (t=0,…,N−1).

By Proposition 7(ii), for every t=0,…,N−1 and j=1,…,d, there exists C _t,j >0 such that, for every positive integer n _t,j , there exist $\tau_{t,j,1}, \ldots,\tau_{t,j,n_{t,j}} \in \mathbb {R}^{d}$, $b_{t,j,1}, \ldots, b_{t,j,n_{t,j}}>0$, and $\delta_{t,j,1}, \ldots, \delta_{t,j,n_{t,j}} \in \mathbb {R}$ for which

$$ \sup_{\bar{y}_t \in \bar{Y}_t} \Biggl\vert g_{t,j}(\bar{y}_t)-\sum_{i=1}^{n_{t,j}} \delta_{t,j,i} \exp \biggl(-\frac{\|\bar{y}_t-\tau_{t,j,i}\|^2}{b_{t,j,i}} \biggr) \Biggr\vert \leq \frac{C_{t,j}}{\sqrt{n_{t,j}}} . $$

(70)

By (16), for every continuous function $\gamma_{t}: \bar{Y}_{t} \to \mathbb {R}^{d}$ we have

$$ \|\gamma_t\|_{{\mathrm{sup}}(\bar{Y}_t)} \leq \sqrt{\sum_{j=1}^d \Bigl( \sup_{\bar{y}_t \in \bar{Y}_t} \bigl|g_{t,j}(\bar{y}_t)\bigr| \Bigr)^2}. $$

(71)

Since $g_{t}(\bar{y}_{t}) \in X_{t+1}$ and $\operatorname {Prj}_{\mathcal {D}_{t}(x_{t},z_{t})}$ is a nonexpansive map, from (70) and (71) we get

$$\begin{aligned} \sup_{\bar{y}_t \in \bar{Y}_t} \Biggl| &g_{t}(\bar{y}_t)-\operatorname {Prj}_{\mathcal {D}_{t}(x_t,z_t)} \Biggl\{ \sum_{i=1}^{n_{t,1}} \delta_{t,1,i} \exp \biggl(-\frac{\|\bar{y}_t-\tau_{t,1,i}\|^2}{b_{t,1,i}} \biggr), \ldots, \\ & \sum_{i=1}^{n_{t,d}} \delta_{t,d,i} \exp \biggl(-\frac{\|\bar{y}_t-\tau_{t,d,i}\|^2}{b_{t,d,i}} \biggr) \Biggr\} \Biggr| \leq \sqrt{\sum_{j=1}^d \frac{C_{t,j}^2}{n_{t,j}}} . \end{aligned}$$

(72)

Hence, for every t=0,…,N−1, there exists ${\tilde{g}}_{t} \in \mathcal {G}(n_{t,1},\ldots,n_{t,d}; \bar{Y}_{t}, X_{t+1})$ such that

$$ \bigl\Vert g_{t}(\bar{y}_t)-{\tilde{g}}_t(\bar{y}_t) \bigr\Vert _{{\mathrm{sup}}(\bar{Y}_t)} \leq \sqrt{\sum_{j=1}^d \frac{C_{t,j}^2}{n_{t,j}}} . $$

We conclude by taking ${\varepsilon}_{t} = \sqrt{\sum_{j=1}^{d} \frac{C_{t,j}^{2}}{n_{t,j}}}$ in (69).  □

Proofs of Theorems 11 and 12

They are along the lines of the proofs of Theorems 9 and 10, resp., by exploiting Proposition 8 instead of Proposition 7. □

Proof of Proposition 14

We first derive some conditions on the sets A _t,j , then we show that the budget constraints (46) are satisfied if and only if the sets A _t,j are chosen as in Assumption 13 (or are suitable subsets).

As the labour incomes y _t,j and the stochastic interest rates r _t,j are bounded from above, for t=1,…,N we have

$$a_{t,j} \leq a_{0,j}^\mathrm{max} \prod _{k=0}^{t-1}\bigl(1+r_{k,j}^\mathrm{max} \bigr) + \sum_{i=0}^{t-1} y_{i,j} \prod_{k=i}^{t-1}\bigl(1+r_{k,j}^\mathrm{max} \bigr)=a_{t,j}^\mathrm{max} $$

(the upper bound is achieved when all the r _t,j are equal to $r_{t,j}^{\mathrm{max}}$ and all the consumptions c _t,j are equal to 0), so the corresponding feasible sets A _t,j are bounded from above by $a_{t,j}^{\mathrm{max}}$. Boundedness from below of each A _t,j follows from the budget constraints (46), which for c _k,j =0 (k=t,…,N) are equivalent to

$$ a_{N,j} \geq -y_{N,j} $$

(73)

for t=N and

$$a_{t,j} \prod_{k=t}^{N-1} (1+r_{k,j}) + \sum_{i=t}^{N-1} y_{i,j} \prod_{k=i}^{N-1} (1+r_{k,j}) + y_{N,j} \geq 0, $$

i.e.,

$$ a_{t,j} \geq -\frac{\sum_{i=t}^{N-1} y_{i,j} \prod_{k=i}^{N-1} (1+r_{k,j}) + y_{N,j}}{\prod_{k=t}^{N-1} (1+r_{k,j} )} $$

(74)

for t=0,…,N−1. For k=t,…,N−1, one can show that the maximum value of the right-hand side in the lower bound (74) occurs when each r _k,j is equal to $r_{k,j}^{\mathrm{max}}$ (indeed, by computing its partial derivatives with respect to r _k,j it is easy to see that they are nonnegative). So, in order to satisfy the budget constraints (46) independently of the specific sequence of stochastic interest rates, the constraints (73) and

$$ a_{t,j} \geq -\frac{\sum_{i=t}^{N-1} y_{i,j} \prod_{k=i}^{N-1} (1+r_{k,j}^\mathrm{max}) + y_{N,j}}{\prod_{k=t}^{N-1} (1+r_{k,j}^\mathrm{max} )}, \quad t=0,\ldots,N-1, $$

(75)

have to be satisfied. Then the maximal sets A _t that satisfy the budget constraints (46) have the form described in Assumption 13.  □

Proof of Proposition 16

• (a) Assumption 2(i), (ii), and (iii). These follow immediately by the choices of A _t and R _t , and by Assumption 15(i). By construction, the sets $\bar{A}_{t}$ are compact, convex, and have nonempty interiors, since they are Cartesian products of nonempty closed intervals. The same holds for the sets $\bar{D}_{t}$, since by (48) they are the intersections between $\bar{A}_{t} \times R_{t} \times \bar{A}_{t+1}$ and the sets D _t , which are compact, convex, and have nonempty interiors, too.

• (b) Assumption 2(iv). Recall that for Problem OCU^d we have

  $$h_N(a_N)= u (a_N+y_N ). $$

  Then, $h_{N} \in \mathcal {C}^{m}(\bar{A}_{N})$ and by Assumption 15(ii) is α _N -concave with α _N =α.

• (c) Assumption 2(v). Recall that for Problem OCU^d and t=0,…,N−1, we have

  $$ h_t(a_t,a_{t+1})=u \biggl(\frac{(1+r_t) \circ\,(a_t+y_t)-a_{t+1}}{1+r_t} \biggr)+\sum_{j=1}^d v_{t,j}(a_{t,j}). $$

  Then, $h_{t} \in \mathcal {C}^{m}(\bar{D}_{t})$ by Assumption 15(ii), (iii). As u and v _t,j are twice continuously differentiable, the second part of Assumption 2(v) means that there exists some α _t >0 such that, for each fixed r _t , the function

  $$u \biggl(\frac{(1+r_t) \circ\,(a_t+y_t)-a_{t+1}}{1+r_t} \biggr)+\sum_{j=1}^d v_{t,j}(a_{t,j})+ \frac{1}{2}\alpha_t \|a_t\|^2 $$

  has negative semi-definite Hessian with respect to the variables a _t and a _t+1. Assumption 15(ii) and straightforward computations show that, for fixed r _t , the function

  $$u \biggl(\frac{(1+r_t) \circ\,(a_t+y_t)-a_{t+1}}{1+r_t} \biggr) $$

  has negative semi-definite Hessian. By Assumption 15(iii), for each j=1,…,d and α _t,j ∈(0,β _t,j ], $v_{t,j}(a_{t,j})+ \frac{1}{2}\alpha_{t,j} a_{t,j}^{2}$ has negative semi-definite Hessian, too. So, Assumption 2(v) is satisfied for every α _t ∈(0,min _j=1,…,d {β _t,j }].

• (d) Assumption 2(vi). This is Assumption 15(i), with the obvious replacements of X _t and D _t .  □