q-analoge Van derMonde identity

I have the following problem from Van Lint and Wilson's book on combinatorics and have no idea how to solve it. The problem is the following:

Determine the exponents $e_i$ (the may be functions of $m$,$n$, and $k$ as well as $i$ ) so that the following identity is valid: $${n + m\choose k}_q = \sum_{i = 0}^{k}q^{e_i}{n\choose i}_q{m\choose k-i }_q$$

Could anyone give me a hint on how to solve this problem? I understand that the left hand side is counting the number of $k$-dimensional subspaces of a vector space of dimension $n + m$. In the left hand side, the two q-binomial terms are trying to count the number of subspaces of dimension $k$ by counting first the number of subspaces of dimension $i$ from a vector space of dimension $n$ and the the number of subspaces of dimension $k-i$ of a vector space of dimension $m$, however, I don't understand how to think of the $q^{e_i}$ term.

• Try for $k=0$ or $1$, and some simple $m,n$. I dont know the theory of q-analogs but try to start from the more simple cases to see if you can see a pattern. – Masacroso Sep 25 '16 at 3:28

Here's a $\mathbb{F}_q$-linear algebraic interpretation of Chu-Vandermonde convolution's $q$-analog:
$$\left[\begin{array}{c} m+n \\ k \end{array}\right]_q=\sum_{r+s=k} q^{r(n-s)} \left[\begin{array}{c} m \\ r \end{array}\right]_q\left[\begin{array}{c} n \\ s \end{array}\right]_q .$$
Identify $\mathbb{F}_q^{\,m+n}$ with $\mathbb{F}_q^{\,m}\times\mathbb{F}_q^{\,n}$. Given any subspace $V$ (of dimension $k$), its elements look like $(x,y)$ in this direct product, and if we have $(x,a),(x,b)\in V$ then their difference $(0,a-b)\in V$ too. In light of this, define $X$ to be the image of $V$ under the projection to the first factor $\mathbb{F}_q^{\,m}$ and define $Y$ to be the intersection of $V$ with $0\times\mathbb{F}_q^{\,n}$, i.e. the kernel of the projection's restriction to $V$. For every element $x\in X$ there are a range of $v\in\mathbb{F}_q^{\,n}$ for which $(x,v)\in V$, and all of them are equal modulo the subspace $Y$. Thus, $V$ induces a linear map $X\to \mathbb{F}_q^{\,n}/Y$.
Conversely, given any pair of subspaces $X\le\mathbb{F}_q^{\, m}$, $Y\le\mathbb{F}_q^{\,n}$ and linear map $\phi:X\to \mathbb{F}_q^{\,n}/Y$, we can reconstruct $V$ as the union of cosets $\bigcup_{x\in X}\big(x+\phi(x)\big)$. If $\dim X=r$ and $\dim Y=s$, then the number of such linear maps equals the number of $(n-s)\times r$ matrices over $\mathbb{F}_q$, which is $q^{r(n-s)}$.