4 tweaked title.

# Have derivatives of determinants along 1-psg's ever been 'coherently' computed viaJacobi'sformula?

3 clarified notation

Suppose $\mathfrak{p}$ denotes all the symmetric matrices in $\mathfrak{sl}_{2n} \mathbb{R}$.

Then for each parameterized 1-dimensional linear subspace $\xi=\xi(t)$ of $\mathfrak{p}$ we get a 1-parameter subgroup $e^{\xi(t)}$ in $SL_{2n}\mathbb{R}$.

Now let us take some collection of $n$ linearly independant vectors $x_1, \ldots, x_n$ in $\mathbb{R}^{2n}$, and let $X$ be the $(2n \times n)$ matrix with $x_i$'s as columns.

Setting $\Xi(t)={}^tX \Xi(t):={}^tX e^{\xi(t)}X$ consider now the following problem: for along which lines $\xi$ in $\mathfrak{p}$ is the function $$det\Xi(t)$$ \tilde{\Xi}(t):=det(\Xi(t))$increasing? There is at least one obvious strategy: we shall determine those$\xi$for which the differential$d \Xi$tilde {\Xi}$ is positive.

As one tool, there is the so-called Jacobi formula. This very general formula tells us $$d \Xi=tr(adj~( tilde{\Xi}=tr(adj~( \Xi(t))~~ d \Xi).$$

So in one sense, the Jacobi formula 'computes' our derivative. However it does nothing for us (it is a 'shallow' formula). It does nothing because I imagine there is no individual in the history of the world who could describe what the 'adjugate' of a matrix 'is' (a definition, by itself, gives no images).

But I would like to be wrong on this final point. And so my own question: can anybody refer me to an instance in the world/literature/experience where either the differential of a determinant has 'fallen out' or where Jacobi's formula has yielded something tangible?

2 corrected notation

Suppose $\mathfrak{p}$ denotes all the symmetric matrices in $\mathfrak{sl}_{2n} \mathbb{R}$.

Then for each parameterized 1-dimensional linear subspace $\xi=\xi(t)$ of $\mathfrak{p}$ we get a 1-parameter subgroup $e^{\xi(t)}$ in $SL_{2n}\mathbb{R}$.

Now let us take some collection of $n$ linearly independant vectors $x_1, \ldots, x_n$ in $\mathbb{R}^{2n}$, and let $X$ be the $(2n \times n)$ matrix with $x_i$'s columns.

Consider

Setting $\Xi(t)={}^tX e^{\xi(t)}X$ consider now the following problem: for which lines $\xi$ in $\mathfrak{p}$ is the function $$\Xi(t) = det ({}^tX e^{\xi(t)} X)$$ $det\Xi(t)$$increasing? There is at least one obvious strategy: we shall determine those \xi for which the differential d \Xi is positive. As one tool, there is the so-called Jacobi formula. This very general formula tells us$$d \Xi=tr(adj~( \Xi(t))~~ d \Xi).$\$

So in one sense, the Jacobi formula 'computes' our derivative. However it does nothing for us (it is a 'shallow' formula). It does nothing because I imagine there is no individual in the history of the world who could describe what the 'adjugate' of a matrix 'is' (a definition, by itself, gives no images).

But I would like to be wrong on this final point. And so my own question: can anybody refer me to an instance in the world/literature/experience where either the differential of a determinant has 'fallen out' or where Jacobi's formula has yielded something tangible?

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