Let $F(n, l, i, j)$ be the cardinality of the set
\begin{eqnarray*}
\{(k_1, \cdots, k_n)\in\mathbb{Z}^{\oplus n}|0\leq k_r\leq l-1\text{ for }1\leq r\leq n\text{, }k_1+\cdots+k_n=lj-i\}.
\end{eqnarray*}
Define an $n\times n$ matrix $M$ by 
\begin{eqnarray*}
M_{ij}=(-1)^{i+j}F(n, l, i, j).
\end{eqnarray*}
In fact $M$ is related to the Adams operations on $U(n)$, and I can show using algebraic topology that the eigenvalues are $1, l, l^2, \cdots, l^{n-1}$. 

**Question**: Are there more elementary ways to show this?