Let $\sigma$ be the Lebesgue measure on the unit sphere $\mathbb S^{n-1}$ of $\mathbb R^n$.
Let $\Sigma$ be a semi-definite positive symmetric matrix in dimension $n$.
Is it possible to get a closed-form expression for the following integral:
$$\int_{u \in \mathbb S^{n-1}} \left(u'\Sigma u\right)^{\frac 12} \ \sigma(du)$$