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Is there a vector field $X$ on $\operatorname{M}_n(\mathbb{R})$ or $\operatorname{GL}(n,\mathbb{R})$ with the following condition: $$\begin{cases} X\cdot \operatorname{trace}=\operatorname{Det} \\X\cdot \operatorname{Det}=-\operatorname{trace} \end{cases}$$ where $\operatorname{Det}$ is determinant?

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    $\begingroup$ What do you mean with $X.trace$ and $X.Det$? $\endgroup$
    – Wojowu
    Commented Jan 14, 2020 at 15:40
  • $\begingroup$ @Wojowu It is the derivative of Det along solution curves of vector field $X$. $\endgroup$ Commented Jan 14, 2020 at 17:54

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$\DeclareMathOperator\trace{trace}\DeclareMathOperator\Det{Det}$No. Let $\sum_{i,j}x_{ij}(M)\frac\partial{\partial m_{ij}}$ be this vector field. These conditions amount to writing $$\sum_ix_{ii}(M)=\det M,\qquad\sum_{ij}x_{ij}(M)\hat m_{ij}=-{\rm Tr}M,$$ where $\hat M$ is the cofactor matrix. Take $M=a I_n$, for which $\hat M=a^{n-1}I_n$. One must have $$\sum_ix_{ii}(a I_n)=a^n,\qquad a^{n-1}\sum_{i}x_{ii}(a I_n)=-na,$$ which is inconsistent when $a^{2n-1}\ne-an$.

Edit. Since both the trace and the determinant are homogeneous polynomial, you could be interested in a vector field in which the conditions are consistent with homogeneity. Then its coordinates would be themselves homogeneous polynomials in the entries of $M$. For instance, looking for a vector field $X$ such that $$X\cdot \trace=\Det,\qquad X\cdot \Det=-(\trace)^{2n-1}$$ makes sense. The last righ-hand side could be replaced by $- \trace^{n-1}\Det$.

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    $\begingroup$ Does a vector field satisfying your modified condition exist? $\endgroup$
    – LSpice
    Commented Jan 14, 2020 at 17:09
  • $\begingroup$ Thank you for your answer and the very interesting modification of the question $\endgroup$ Commented Jan 14, 2020 at 17:55
  • $\begingroup$ @LSpice that is a very good point to think. Thanks! $\endgroup$ Commented Jan 14, 2020 at 17:56

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