Let $n$ be an integer and $F$ be a field. Nilpotent matrices $N\in \mathrm M_n(F)$ are those matrices for which there exists an integer $p$ with $N^p=0$. Their characteristic polynomial is $\chi_N(T)=T^n$, and they satisfy $N^n=0$, which shows that the set $\mathscr N_n$ of nilpotent matrices is an algebraic variety. The equation $N^n=0$ is homogeneous of degree $n$, so that $\mathscr N_n$ is a cone.
The classification of nilpotent matrices is an intermediate step in the theory of Jordan decomposition: In an adequate basis, a nilpotent matrix can be written as a diagonal block matrix of “basic” nilpotent matrices, $p \times p$ matrices of the form \[ \begin{pmatrix} 0 & 0 & \dots & 0 & 0 \\ 1 & 0 & & & \vdots \\ 0 & 1 & \ddots & & 0 \\ \vdots & \ddots & \ddots & \ddots & 0 \\ 0 & & 0 & 1 & 0\end{pmatrix} \] whose minimal polynomial is $T^p$. The sum of the sizes of these blocks is $n$ and in this way, it is associated with any $n\times n$ nilpotent matrix a partition $\pi$ of~$n$. It is known that two nilpotent matrices are conjugate if and only if they are associated with the same partition. For any partition $\pi$ of~$n$, let us denote by $J_\pi$ the corresponding matrix whose sizes of blocks are arranged in increasing order, and $\mathscr N_\pi$ the set of nilpotent matrices that are associated with the partition $\pi$.
The theorem of Fine and Herstein (1958)
Having to teach “agrégation” classes made me learn about a classic combinatorial result: counting the number of nilpotent matrices when $F$ is a finite field.
Theorem (Fine, Herstein, 1958). — Let $F$ be a finite field with $q$ elements. The cardinality of $\mathscr N_n(F)$ is $q^{n^2-n}$. Equivalently, the probability that an $n\times n$ matrix with coefficients in $F$ be nilpotent is $q^{-n}$.The initial proof of this results relies on the action of $\GL_n(F)$ on $\mathscr N_n(F)$: we recalled that the orbits correspond with the partitions of $n$, hence a decomposition \[ \Card(\mathscr N_n(F)) = \sum_{\pi} \Card(\mathscr N_\pi(F)). \] We know that $\mathscr N_\pi(F)$ is the orbit of the matrix $J_\pi$ under the action of $\GL_n(F)$. By the classic orbit-stabilizer formula, one thus has \[ \Card(\mathscr N_\pi(F)) = \frac{\Card(\GL_n(F))}{\Card(C_\pi(F))}, \] where $C_\pi(F)$ is the set of matrices $A\in\GL_n(F)$ such that $AJ_\pi=J_\pi A$. The precise description of $C_\pi(F)$ is delicate but their arguments go as follow.
They first replace the group $C_\pi(F)$ by the algebra $A_\pi(F)$ of all matrices $A\in\mathrm M_n(F)$ such that $AJ_\pi=J_\pi A$. For any integer, let $m_i$ be the multiplicity of an integer $i$ in the partition $\pi$, so that $n=\sum i m_i$. The block decomposition of $J_\pi$ corresponds with a decomposition of $F^n$ as a direct sum of invariant subspaces $V_i$, where $V_i$ has dimension $i m_i$. In fact, $V_1=\ker(J_\pi)$, $V_1\oplus V_2=\ker(J_\pi^2)$, etc. This shows that $A_\pi(F)$ is an algebra of block-triangular matrices. Moreover, the possible diagonal blocks can be shown to be isomorphic to $\mathrm M_{m_i}(F)$. In other words, we have a surjective morphism of algebras \[ A_\pi(F) \to \prod_i \mathrm M_{m_i}(F), \] whose kernel consists of nilpotent matrices. In particular, the proportion of invertible elements in $A_\pi(F)$ is equal to the proportion of invertible elements in the product $\prod_i \mathrm M_{m_i}(F)$.
Ultimately, Fine and Herstein obtain an explicit sum over the set of partitions of $n$ which they prove equals $q^{n^2-n}$, after an additional combinatorial argument.
Soon after, the theorem of Fine and Herstein was given easier proofs, starting from Gerstenhaber (1961) to Kaplansky (1990) and Leinster (2021).
A proof
The following proof is borrowed from Caldero and Peronnier (2022), Carnet de voyage en Algébrie. It can be seen as a simplification of the proofs of Gerstenhaber (1961) and Leinster (2021).
Let us start with the Fitting decomposition of an endomorphism $u\in \mathrm N_n(F)$: the least integer $p$ such that $\ker(u^p)=\ker(u^{p+1})$ coincides with the least integer $p$ such that $\im(u^p)=\im(u^{p+1})$, and one has $F^n=\ker(u^p)\oplus \im(u^p)$. The subspaces $N(u)=\ker(u^p)$ and $I(u)=\im(u^p)$ are invariant under $u$, and $u$ acts nilpotently on $\ker(u^p)$ and bijectively on $\im(u^p)$. In other words, we have associated with $u$ complementary subspaces $N(u)$ and $I(u)$, a nilpotent operator of $N(u)$ and an invertible operator on $I(u)$. This map is bijective.
For any integer $d$, let $\nu_d$ be the cardinality of nilpotent matrices in $\mathrm M_d(F)$, and $\gamma_d$ be the cardinality of invertible matrices in $\mathrm M_d(F)$. Let also $\mathscr D_d$ be the set of all pairs $(K,I)$, where $K$ and $I$ are complementary subspaces of dimensions $d$, $n-d$ of $F^n$. We thus obtain \[ n^2 = \sum_{(K,I)\in\mathscr D_d} \nu_d \cdot \gamma_{n-d}. \] We need to compute the cardinality of $\mathscr D_d$. In fact, given one pair $(K,I)\in\mathscr D_d$, all other are of the form $(g\cdot K,g\cdot I)$, for some $g\in\GL_n(F)$: the group $\GL_n(F)$ acts transitively on $\mathscr D_d$. The stabilizer of $(K,I)$ can be identified with $\GL_d(F)\times \GL_{n-d}(F)$. Consequently, \[ \Card(\mathscr D_d) = \frac{\Card(\GL_n(F))}{\Card(\GL_d(F)\Card(\GL_{n-d}(F))} = \frac{\gamma_n}{\gamma_d \gamma_{n-d}}. \] We thus obtain \[ q^{n^2} = \sum_{d=0}^n \frac{\gamma_n}{\gamma_d \gamma_{n-d}} \nu_d \gamma_{n-d} = \gamma_n \sum_{d=0}^n \frac{\nu_d}{\gamma_d}. \] By subtraction, we get \[ \frac{\nu_n}{\gamma_n} = \frac {q^{n^2}}{\gamma_n} - \frac{q^{(n-1)^2}}{\gamma_{n-1}},\] or \[ \nu_n = q^{n^2} - q^{(n-1)^2} \frac{\gamma_n}{\gamma_{n-1}}. \] It remains to compute $\gamma_n$: since an invertible matrix consists of a nonzero vector, a vector which does not belong to the line generated by the first one, etc., we have \[ \gamma_n = (q^n-1) (q^n-q)\dots (q^n-q^{n-1}). \] Then, \[ \gamma_n = (q^n-1) q^{n-1} (q^{n-1}-1)\dots (q^{n-1}-q^{n-2}) = (q^n-1)q^{n-1} \gamma_{n-1}. \] We thus obtain \[ \nu_n = q^{n^2} - q^{(n-1)^2} (q^n-1) q^{n-1} = q^{n^2} - q^{(n-1)^2} q^{2n-1} + q^{(n-1)^2} q^{n-1} = q^{n^2-n}, \] as claimed.
The proof of Leinster (2021)
Leinster defines a bijection from $\mathscr N_n(F)\times F^n$ to $\mathrm M_n(F)$. The definition is however not very canonical, because he assumes given, for any subspace $V$ of $F^n$, a basis of $V$.
Take a pair $(u,x)$, where $u\in\mathscr N_n(F)$ and $x\in F^n$ and consider the subspace $V_x=\langle x,u(x),\dots\rangle$, the smallest $u$-invariant subspace of $F^n$ which contains $x$. To describe $u$, we observe that we know its restriction to $V_x$, and we need to describe it on the chosen complementary subspace $V'_x$.
To that aim, we have to give ourselves an endomorphism $u'_x$ of $V'_x$ and a linear map $\phi_x\colon V'_x\to V_x$. Since we want $u$ to be nilpotent, it is necessary and sufficient to take $u'_x$ nilpotent.
Instead of considering $\phi_x\colon V'_x\to V_x$, we can consider the map $y\mapsto y+\phi_x(y)$. Its image is a complement $W_x$ of $V_x$ in $F^n$, and any complement can be obtained in this way. The nilpotent endomorphism $u'_x$ of $V'_x$ transfers to a nilpotent endomorphism $w_x$ of $W_x$.
All in all, what the given pair $(u,x)$ furnishes is a subspace $V_x$ with a basis $(x_1=x,x_2=u(x),\dots)$, a complement $W_x$, and a nilpotent endomorphism $w_x$ of $W_x$. This is more or less what the Fitting decomposition of an endomorphism gives us! Recall that $V_x$ was assumed to have been given a basis $(e_1,\dots,e_p)$. There exists a unique automorphism of $V_x$ which maps $e_i$ to $u^{i-1}(x)$ for all $i$. In other words, we have a pair of complementary subspaces $(V_x,W_x)$, a linear automorphism of $V_x$, and a nilpotent automorphism of $W_x$. By the Fitting decomposition, these data furnish in a bijective way an endomorphism of $F^n$, and that concludes the proof.
A remark about motivic integration
The framework of motivic integration suggests to upgrade these combinatorial results into equalities valid for all field $F$, which hold in the Grothendieck ring of varieties $\KVar_F$. As an abelian group, it is generated by symbols $[X]$, for all algebraic varieties $X$ over $F$, with relations $[X]=[Y]+[X\setminus Y]$, whenever $Y$ is a closed subvariety of $X$. The ring structure is defined so that the formula $[X]\cdot[Y]=[X\times Y]$ for all algebraic varieties $X$ and $Y$ over $F$.
By construction of this ring, equalities $[X]=[Y]$ in $\KVar_F$ imply that many invariants of $X$ and $Y$ coincide. In particular, when $F$ is a finite field, they will have the same number of points.
The question is thus to compute the class $[\mathscr N_n]$ of the variety $\mathscr N_n$, for any field $F$. The proofs that I described above can be more or less transferred to this context and imply the following theorem. We denote by $\mathbf L\in \KVar_F$ the class of the affine line $\mathbf A^1$.
Theorem. — One has an equality $[\mathscr N_n] \mathbf L^n = \mathbf L^{n^2}$ in the localization of the Grothendieck ring $\KVar_F$ by the element $(\mathbf L-1)\dots(\mathbf L^{n-1}-1)$.
The following question is then natural. (I have not thought about it at all.)
Question. — Does one have $[\mathscr N_n]=\mathbf L^{n^2-n}$ in $\KVar_F$?
