Lemma 10.15.5. Let $R$ be a ring. Let $n \geq m$. Let $A$ be an $n \times m$ matrix with coefficients in $R$. Let $J \subset R$ be the ideal generated by the $m \times m$ minors of $A$.

1. For any $f \in J$ there exists a $m \times n$ matrix $B$ such that $BA = f 1_{m \times m}$.

2. If $f \in R$ and $BA = f 1_{m \times m}$ for some $m \times n$ matrix $B$, then $f^ m \in J$.

Proof. For $I \subset \{ 1, \ldots , n\}$ with $|I| = m$, we denote by $E_ I$ the $m \times n$ matrix of the projection

$R^{\oplus n} = \bigoplus \nolimits _{i \in \{ 1, \ldots , n\} } R \longrightarrow \bigoplus \nolimits _{i \in I} R$

and set $A_ I = E_ I A$, i.e., $A_ I$ is the $m \times m$ matrix whose rows are the rows of $A$ with indices in $I$. Let $B_ I$ be the adjugate (transpose of cofactor) matrix to $A_ I$, i.e., such that $A_ I B_ I = B_ I A_ I = \det (A_ I) 1_{m \times m}$. The $m \times m$ minors of $A$ are the determinants $\det A_ I$ for all the $I \subset \{ 1, \ldots , n\}$ with $|I| = m$. If $f \in J$ then we can write $f = \sum c_ I \det (A_ I)$ for some $c_ I \in R$. Set $B = \sum c_ I B_ I E_ I$ to see that (1) holds.

If $f 1_{m \times m} = BA$ then by the Cauchy-Binet formula (72) we have $f^ m = \sum b_ I \det (A_ I)$ where $b_ I$ is the determinant of the $m \times m$ matrix whose columns are the columns of $B$ with indices in $I$. $\square$

Comment #3577 by Herman Rohrbach on

If the philosophy of this section of not using any results other than those listed in the basic notions is to be followed, perhaps the Cauchy-Binet formula should either be included in the basic notions, or stated in this section? I know it's not very important, but I found it worth to at least mention it.

Comment #3701 by on

Thanks. I added the Cauchy-Binet formula to the list of basic notions here.

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