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Let $\mathfrak{g}_0$ be a noncompact simple Lie algebra with Cartan decomposition $\mathfrak{g}_0=\mathfrak{k}_0+\mathfrak{p}_0$. Write $\mathfrak{g}=\mathfrak{k}+\mathfrak{p}$ for the complexification. Denote by $K_\mathbb{C}$ the analytical Lie group corresponding to $\mathfrak{k}$. Then $K_\mathbb{C}$ acts on $\mathfrak{p}$. If $\mathfrak{g}_0$ is not of Hermitian type, then $K_\mathbb{C}$ acts irreducibly on $\mathfrak{p}$. Fix a Cartan subalgebra and a positive system for $K_\mathbb{C}$, and let $X$ be a highest weight vector in $\mathfrak{p}$ as the $K_\mathbb{C}$-representation. It is well known that the minimal orbit of the $K_\mathbb{C}$-action on $\mathfrak{p}$ is $K_\mathbb{C}\cdot X$.

Is there any other canonical representative of this minimal orbit? Namely, is there any other element $Y\in\mathfrak{p}$ (which is easy to write down) such that $K_\mathbb{C}\cdot Y=K_\mathbb{C}\cdot X$?

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    $\begingroup$ Won't anything in the orbit of a highest weight vector simply be a highest weight vector for a different choice of CSA and ordering? $\endgroup$
    – Callum
    Commented Aug 14, 2023 at 13:22
  • $\begingroup$ @Callum You are right. But if we fix a Cartan subalgebra and a positive system for $K$, is there any other vector in the minimal orbit which is easily expressed by the given weight vectors? $\endgroup$
    – Hebe
    Commented Aug 15, 2023 at 2:33
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    $\begingroup$ In general it will pass through some other weight spaces (every one in the Weyl orbit of the highest weight - equivalently every one which is a highest weight if we change the ordering) but these won't be easier to represent then the highest weight $\endgroup$
    – Callum
    Commented Aug 15, 2023 at 8:35

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