# volume form in a symmetric space of real rank one

I want to compare the two canonical volume forms on a noncompact symmetric space fo real rank one. The first one is the volume form induced by the Riemannian structure given by the Killing form restricted on $\mathfrak p$. The second one is $d\bar g$ induced by $dg =\gamma(a_t)\ dk_1 \ da \ dk_2$. It is known that $$dx=c \ d\bar g,\qquad \text{with c\in\mathbb R.}$$ What is $c$?

Now, a more detailed exposition. Let $G$ be a connected semisimple Lie group of real rank one and finite center. Let:

• $\mathfrak g=\mathfrak k \oplus \mathfrak p$ a Cartan decomposition;
• $G=NAK$ be an Iwasawa decomposition of $G$;
• $\mathfrak g=\mathfrak k\oplus \mathfrak a\oplus \mathfrak n$ the corresponding decomposition of $\mathfrak g$;
• $B$ Killing form on $\mathfrak g$,
• $\alpha$ the simple root ($G$ has real rank one);
• $p=\dim \mathfrak n_\alpha$, $q=\dim \mathfrak n_{2\alpha}$;
• $H_\alpha\in\mathfrak a$ with $\alpha(H_\alpha)=1$;
• $A^+=\{ a_t:=\exp(t H_\alpha) : t>0 \}$,
• $M$ the centralizer of $A$ in $K$.

Let $X=G/K$ be the symmetric space with the Riemannian structure induced by $B$ over $\mathfrak p$. Let $dx$ the volume form induced by this Riemannian structure.

Let $dg$ (resp. $d\bar g$) be the Haar measure on $G$ (resp. $G/K$) such that $$dg = \gamma(a_t)\ dk_1 \ da \ dk_2$$ on $KA^+K$, where $\gamma(a_t) = (e^t-e^{-t})^p (e^{2t}-e^{-2t})^q=2^{p+q}(\sinh t)^p(\sinh 2t)^q$, $da=dt$ and $dk$ is the Haar measure on $K$ normalized so that $K$ has volume 1.

Thanks.-.