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Orbits/CurrentWriting/sections/05_SocialPlanner.tex

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\documentclass[../Main.tex]{subfiles}
\graphicspath{{\subfix{Assets/img/}}}
\begin{document}
The Social (Fleet) Planner's problem can be written in the belman form as:
\begin{align}
W(\vec s_t, D_t) =& \max_{\vec x_t} \left[
\left(\sum^N_{i=1} u^i(\vec s_t, D_t) - F(x^i_t) \right)
+ \beta \left[ W(\vec s_{t+1}, D_{t+1}) \right]\right] \notag \\
&\text{subject to:} \notag \\
& s^i_{t+1} = (1-l^i(\vec s_t, D_t))s^i_t +x^i_t ~~~ \forall i \notag \\
& D_{t+1} = (1-\delta)D_t + g(D_t)
+ \gamma \sum^N_{i=1} l^i(\vec s_t, D_t)
+ \Gamma \sum^N_{i=1} x^i_t
\end{align}
The resulting $N$ optimality conditions are:
\begin{align}
0 =& -\der{F(x^i_t)}{s^i_t}{}
+ \beta \left[
\nabla_{\vec s_{t+1}, D_{t+1}} W(\vec s_{t+1}, D_{t+1})
\cdot
\nabla_{\vec s_{t}, D_{t}}[\vec s_{t+1} ~ D_{t+1}]
\right]
~~\forall~~i \\
0 =& -\vec f +\beta \left[B\cdot \nabla W_{t+1} \right]
\end{align}
And the $N+1$ envelope conditions are:
\begin{align}
\nabla_{\vec s_{t}, D_{t}} W(\vec s_t, D_t) =&
\sum^N_{i=1} \nabla_{\vec s_{t}, D_{t}} u^i(\vec s_t, D_t)
%- \der{}{x^i_t}{}F(x^i_t) \nabla_{\vec s_{t}, D_{t}}x^i_t %This equals zero due to the envelope theorem
\notag \\
&+ \beta \left[ \nabla_{\vec s_{t+1}, D_{t+1}} W(\vec s_{t+1}, D_{t+1})
\cdot \nabla_{\vec s_{t}, D_{t}} [\vec s_{t+1} ~ D_{t+1}]
\right] \\
\nabla W_t =& \vec U + \beta \left[C \cdot \nabla W_{t+1} \right]
\end{align}
\end{document}