3 added 13 characters in body

As David says $Q$ is the unipotent radical of $P$. The subgroup $T$ is a preimage of the Weyl group $W$ of the group $G_i\cong GL(V_i/ V_{i+1})$. This group $T$ looks a direct product of $Q$ with a big chunk of a Levi complement of $P$. The Levi complement is a direct product isomorphic to $G_1\times\cdots \times G_k$; to obtain the group $T$, you replace the $i$-th factor by the normalizer $N$ of a maximal split torus $T_0$ of $G_i$.

This is, in fact, the typical way to realize the Weyl group of $G_i$: G_i$--$W$is isomorphic to the quotient$N/T_0$where$N$is the normalizer of a maximal split torus$T_0$in$G_i$-- but this is effectively the same thing as your method of fixing a specific basis of$V_i/V_{i+1}$. The Weyl group rears its head in lots of different ways (most especially as a Coxeter group related to the Dynkin diagram of$G_i$) so this is certainly not the only way to realise it. I don't, however, see any other way to realize realise your group$T$.T$ (although it depends what you mean by `realise'!).

As for references, it depends on what kind of approach you want. If you want a treatment of $GL_n$ as an algebraic group then I recommend anything by Carter or Humphreys, or else there is the book by Borel. All of these people work in much greater generality than $GL_n$ though. If you just want to understand $GL_n$, then standard algebra texts like the one of Jacobson might be your best bet. (I have e-copies of some of these. If you want them, email me.)

2 expanded treatment of T

As David says $Q$ is the unipotent radical of $P$. The subgroup $T$ is (isomorphic to) a preimage of the Weyl group $W$ of the group $G_i\cong GL(V_i/ V_{i+1})$. This group $T$ looks a direct product of $Q$ with a big chunk of a Levi complement of $P$. The Levi complement is a direct product isomorphic to $G_1\times\cdots \times G_k$; to obtain the group $T$, you replace the $i$-th factor by the normalizer of a maximal split torus of $G_i$.

This is, in fact, the typical way to realize the Weyl group of $G_i$: $W$ is as the quotient $N/T$ N/T_0$where$N$is the normalizer of a maximal split torus$T$T_0$ in $G_i$ - this is effectively the same thing as your method of fixing a specific basis of $V_i/V_{i+1}$. The Weyl group rears its head in lots of different ways (most especially as a Coxeter group related to the Dynkin diagram of $G_i$) so this is certainly not the only way to realise it. I don't, however, see any other way to realize your group $T$.

As for references, it depends on what kind of approach you want. If you want a treatment of $GL_n$ as an algebraic group then I recommend anything by Carter or Humphreys, or else there is the book by Borel. All of these people work in much greater generality than $GL_n$ though. If you just want to understand $GL_n$, then standard algebra texts like the one of Jacobson might be your best bet. (I have e-copies of some of these. If you want them, email me.)

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As David says $Q$ is the unipotent radical of $P$. The subgroup $T$ is (isomorphic to) the Weyl group of the group $G_i\cong GL(V_i/ V_{i+1})$.

The typical way to realize the Weyl group is as the quotient $N/T$ where $N$ is the normalizer of a maximal split torus $T$ in $G_i$ - this is effectively the same thing as fixing a specific basis of $V_i/V_{i+1}$. The Weyl group rears its head in lots of different ways (most especially as a Coxeter group related to the Dynkin diagram of $G_i$) so this is certainly not the only way to realise it.

As for references, it depends on what kind of approach you want. If you want a treatment of $GL_n$ as an algebraic group then I recommend anything by Carter or Humphreys, or else there is the book by Borel. All of these people work in much greater generality than $GL_n$ though. If you just want to understand $GL_n$, then standard algebra texts like the one of Jacobson might be your best bet. (I have e-copies of some of these. If you want them, email me.)