Consider the reward function $r(s, a)$ with optimal state-action value function $q_*(s, a)$. What would be the optimal state-action value function of $c r(s, a)$, for $c \in \mathbb{R}$? Would it be $c q_*(s, a)$?
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The Bellman optimality equation is given by
$$q_*(s,a) = \sum_{s' \in \mathcal{S}, r \in \mathcal{R}}p(s',r \mid s,a)(r + \gamma \max_{a'\in\mathcal{A}(s')}q_*(s',a')) \tag{1}\label{1}.$$
If the reward is multiplied by a constant $c > 0 \in \mathbb{R}$, then the new optimal action-value function is given by $cq_*(s, a)$.
To prove this, we just need to show that equation \ref{1} holds when the reward is $cr$ and the action-value is $c q_*(s, a)$.
\begin{align} c q_*(s,a) &= \sum_{s' \in \mathcal{S}, r \in \mathcal{R}}p(s',r \mid s,a)(c r + \gamma \max_{a'\in\mathcal{A}(s')} c q_*(s',a')) \tag{2}\label{2} \end{align}
Given that $c > 0$, then $\max_{a'\in\mathcal{A}(s')} c q_*(s',a') = c\max_{a'\in\mathcal{A}(s')}q_*(s',a')$, so $c$ can be taken out of the $\operatorname{max}$ operator. Therefore, the equation \ref{2} becomes
\begin{align} c q_*(s,a) &= \sum_{s' \in \mathcal{S}, r \in \mathcal{R}}p(s',r \mid s,a)(c r + \gamma c \max_{a'\in\mathcal{A}(s')} q_*(s',a')) \\ &= \sum_{s' \in \mathcal{S}, r \in \mathcal{R}}c p(s',r \mid s,a)(r + \gamma \max_{a'\in\mathcal{A}(s')} q_*(s',a')) \\ &= c \sum_{s' \in \mathcal{S}, r \in \mathcal{R}} p(s',r \mid s,a)(r + \gamma \max_{a'\in\mathcal{A}(s')} q_*(s',a')) \\ q_*(s,a) &= \sum_{s' \in \mathcal{S}, r \in \mathcal{R}} p(s',r \mid s,a)(r + \gamma \max_{a'\in\mathcal{A}(s')} q_*(s',a')) \tag{3}\label{3} \end{align} which is equal to the the Bellman optimality in \ref{1}, which implies that, when the reward is given by $cr$, $c q_*(s,a)$ is the solution to the Bellman optimality equation. Consequently, in this case, the set of optimal policies does not change.
If $c=0$, then \ref{2} becomes $0=0$, which is true.
If $c < 0$, then $\max_{a'\in\mathcal{A}(s')} c q_*(s',a') = c\min_{a'\in\mathcal{A}(s')}q_*(s',a')$, so equation \ref{3} becomes
\begin{align} q_*(s,a) &= \sum_{s' \in \mathcal{S}, r \in \mathcal{R}} p(s',r \mid s,a)(r + \gamma \min_{a'\in\mathcal{A}(s')} q_*(s',a')) \end{align}
which is not equal to the Bellman optimality equation in \ref{1}.
