[Math] Determinant-like expression for non-square matrices

determinantlinear algebramatrices

I'm interested in whether for any real matrix of size $m \times n$ there is a real number with the following properties:

  1. It is a polynomial expression with real coefficients in the entries of the matrix. The expression depends on $m,n$ only.
  2. It is zero precisely when the matrix is not of full rank ($\min\left\{m,n\right\}$).

For square matrices, the determinant has these properties.

If this is a known thing, what is it called and where can I read about it?

Best Answer

There is such a thing, at least over the reals. Suppose $m>n$. Then an $m\times n$ matrix has full rank if and only if it contains an $n\times n$ submatrix of full rank. Let $A$ be an $m\times n$ matrix and let $A_1,\dots,A_N$ be its $n\times n$ submatrices. (The exact value of the number $N$ is irrelevant here; it only depends on $m$ and $n$.) Now let $D(A)=\sum_{k=1}^N\det(A_k)^2$. Clearly $D(A)$ is polynomial in each element since the determinant is, and $D(A)=0$ if and only if none of the $n\times n$ submatrices of $A$ has full rank.

I don't know if such things have been studied or given a name.

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