<s>[INST] Probability of a $1000 \times 1000$ square matrix over $\mathbb{Z}_2$ having full rank There are only two entries, $0$ and $1$, over $\mathbb{Z}_2$. Thus, only $16$ possible $2\times2$ matrices over $\mathbb{Z}_2$, and $6$ of them have full rank:
$$\begin{pmatrix}0&1\\  
1&0\end{pmatrix}  \quad
\begin{pmatrix}1&1\\  
1&0\end{pmatrix}  \quad
\begin{pmatrix}0&1\\  
1&0\end{pmatrix}  \quad
\begin{pmatrix}0&1\\  
1&1\end{pmatrix}  \quad
\begin{pmatrix}1&1\\  
0&1\end{pmatrix}  \quad
\begin{pmatrix}1&0\\  
0&1\end{pmatrix}$$ 
Randomly generate a $n \times n$ matrix over $\mathbb{Z}_2$ (where $n$ is big, say, $1000$). What's the probability that the matrix has full rank?
 [/INST] The general linear group $GL(n,q)$ is the group of invertible $n\times n$ matrices over a field with $q$ elements (note $q=p^k$ for some prime $p$). 
The order of $$|GL(n,q)|=\prod_{k=0}^{n-1}(q^n-q^k)$$
So the probability is $$\dfrac{1}{q^{n^2}}\prod_{k=0}^{n-1}(q^n-q^k)$$
For $n=2$ $q=2$ you get $\frac{1}{2^4}(2^2-2)(2^2-1)=\frac{6}{16}$
</s>