Parabolic Induction in Stages

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$\DeclareMathOperator{\Ind}{Ind}$Let $G$ be a connected, reductive group over a local field $k$.

Let $P_{\ast} = M_{\ast}N_{\ast} \subseteq P = MN$ be parabolic subgroups, standard with respect to a given minimal parabolic, with standard Levi subgroups $M_{\ast} \subseteq M$.

Then $M_{\ast}(N_{\ast} \cap M)$ is a parabolic subgroup of $M$ with unipotent radical $N_{\ast} \cap M$. If $(\pi,V)$ is a representation of $M_{\ast}$, then we can regard $\pi$ as a representation of $P_{\ast}$ by making it trivial on $N_{\ast}$, and form the induced representation $\Ind_{P_{\ast}}^G \pi$. On the other hand, we can regard $\pi$ as a representation of $M_{\ast}(N_{\ast} \cap M)$ by making it trivial on $N_{\ast} \cap M$, and form the induced representation

$$\sigma = \Ind_{M_{\ast}(N_{\ast} \cap M)}^M \pi$$

Then we can extend $\sigma$ to $P$ by making it trivial on $N$, and form

$$\Ind_P^G \sigma$$

I expect that we should be able to identify the representations

$$\Ind_P^G\Ind_{M_{\ast}(N_{\ast} \cap M)}^M \pi = \Ind_{P_{\ast}}^G \pi$$

This initially appears to be an application of the transitivity of induction, but this principle cannot be immediately implied. Are these two representations of $G$ the same? What if we do these by normalized induction? (Taking into account the modulus characters)

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$\DeclareMathOperator{\Hom}{Hom}$I think that this follows from a version of Frobenius reciprocity: if $(\tau,W)$ is a representation of $G$, and $W_N = W/W(N)$ is the Jacquet module of $P$ (where $W(N)$ is the linear span of $w - \tau(n)w : w \in W, n \in N$), then we have

$$\Hom_G(W, \Ind_P^G\pi) = \Hom_M(W_N,\pi)$$

Here $W_N$ inherits the structure of a representation of $M$. We can further restrict this to a representation of the parabolic subgroup $M_{\ast}(N_{\ast} \cap M)$ of $M$ and take the Jacquet module again.

Lemma: The Jacquet module $W_N/[W_N(N_{\ast} \cap M)]$ coincides with $W_{N_{\ast}}$ as a representation of $M_{\ast}$.

Proof: By the third isomorphism theorem, this comes down to showing that the subspace $W_N(N_{\ast} \cap M)$ of $W_N = W/W(N)$ is equal to

$$W_N(N_{\ast} \cap M) = \frac{W(N_{\ast})}{W(N)}$$

It is clear that the left hand side is contained in the right. Now consider a generator $w - \tau(n)w + W(N)$ of the right hand side, for some $w \in W$ and $n \in N_{\ast}$. Since $N_{\ast}$ is the semidirect product of $N$ by $N_{\ast} \cap M$, we can write $n = n_1n_2$ for $n_1 \in N_{\ast} \cap M$ and $n_2 \in N$. Then

$$w - \tau(n)w + W(N) = w - \tau(n_2)w + \tau(n_2)w - \tau(n)w+W(N) = \tau(n_2)w - \tau(n)w + W(N)$$

with $\tau(n_2)w - \tau(n)w = \tau(n_2)w - \tau(n_1)\tau(n_2)w$. So this is in the left hand side. $\blacksquare$

Now we have natural bijections in both variables

$$\begin{equation*} \begin{split} \Hom_G(W,\Ind_P^G \Ind_{M_{\ast}(N_{\ast} \cap M)}^M \pi) & = \Hom_M(W_N,\Ind_{M_{\ast}(N_{\ast} \cap M)}^M \pi) \\ & = \Hom_{M_{\ast}}(W_{N_{\ast}},\pi) \\ & = \Hom_G(W,\Ind_{P_{\ast}}^G\pi) \end{split} \end{equation*}$$

so the functors $\Ind_{P_{\ast}}$ and $\Ind_{M_{\ast}(N_{\ast} \cap M)}^M$ must be naturally isomorphic.

We get the same result if we replace induction by normalized induction. This is on account of the fact that if $\delta_1$ is the modulus character for $M_{\ast}$ on $(N_{\ast} \cap M)$, and $\delta_2$ is the modulus character for $M$ on $N$, then $\delta_1 \delta_2|_{M_{\ast}}$ is the modulus character for $M_{\ast}$ on $N_{\ast}$.