# Stationary phase in spherical integral

I'm trying to find asymptotics for an oscillatory integral on $$\mathbb{S}^{n-1}$$, for which my advisor said I should use stationary phase arguments. The particular, he claims that:

If $$\lambda\gg 1$$, then $$I(\lambda,x) = \int_{\mathbb{S}^{n-1}}(x\cdot y)e^{i\lambda(x\cdot y)}\,d\sigma(y),$$ is $$\mathcal{O}(|x|(\lambda |x|)^{-\frac{n-1}{2}})$$ when $$|x|\geq \lambda^{-1}$$.

When I hear stationary phase, I think of working with operators of the form $$L = \frac{\nabla_y(x\cdot y)}{i\lambda |\nabla_y (x\cdot y)|^2} \cdot \nabla_y,$$ since then $$L^N[e^{i\lambda (x\cdot y)}] = e^{i\lambda (x\cdot y)}$$, for any $$N \geq 1$$, and I can use integration by parts to move these operators over to the $$(x\cdot y)$$ term. However, wouldn't that require that the integral be defined over an $$n$$-dimensional region, rather than an $$(n-1)$$-dimensional surface?

I wouldn't be struggling so much if the gradient $$\nabla_y$$ could be taken in Cartesian coordinates. But we have a surface integral in $$d\sigma(y)$$, meaning that we would need to parametrize our surface with $$n-1$$ parameters, say $$\omega = (\omega_1, \ldots, \omega_{n-1}) \in \Omega \subset \mathbb{R}^{n-1}$$, with any derivates now being taken in these new variables. Specifically $$\nabla_y$$ would generate an $$n$$-vector, while $$\nabla_{\omega}$$ would generate an $$n-1$$-vector. So how should I go about applying the same kind of stationary phase arguments to this new integral? $$I(\lambda, x) = \int_{\Omega} (x \cdot y(\omega)) e^{i\lambda (x\cdot y(\omega))} \,dV(\omega)$$ Now that $$y$$ depends on $$\omega$$, the gradients $$\nabla_\omega (x\cdot y(\omega))$$ become much trickier to get a grasp on. I'm particularly struggling trying to argue how we can find regions where $$|\nabla_\omega (x\cdot y(\omega))| > 0$$, so that our $$L$$ type operators are properly defined.

Am I going about this all wrong? If I can get the big-oh asymptotics I mentioned above in some other way, it doesn't really matter. I just need to prove these results as a lemma to something bigger. Any help is much appreciated!

You have $$I(\lambda, x)=x\cdot\int_{\mathbb S^{n-1}} ye^{i \lambda x\cdot y} d\sigma(y)=x\cdot J(x,\lambda)$$ and you claim that for $$\vert x\vert \lambda \ge 1$$, you have $$J(x,\lambda)=O((\vert x\vert \lambda)^{-\frac{n-1}{2}}).$$ Indeed, using coordinate charts and a finite partition of unity, you are reduced to the case where $$J(x,\lambda)=\int_{\mathbb R^{n-1}} a(z) e^{i\lambda (x'\cdot z+ x_n\sqrt{1- \vert z\vert^2})}dz, \quad\text{a\in C^\infty_0(\mathbb R^{n-1}) supported near 0, x=(x', x_n)\in \mathbb R^{n-1}\times \mathbb R}.$$ Let us set $$\phi(x,z)=x'\cdot z+ x_n\sqrt{1- \vert z\vert^2}$$. We have $$\frac{\partial \phi}{\partial z}= x'-(1- \vert z\vert^2)^{-1/2} z x_n,$$ which vanishes at $$z=0$$ when $$x'=0$$. Then you calculate the Hessian of $$\phi$$ at $$z=0$$ and get $$\phi''_{zz}=-x_n.$$ The stationary phase in $$n-1$$ dimensions gives the sought estimate with $$O((\vert x_n\vert \lambda)^{-\frac{n-1}{2}})$$ (note that you know that $$\vert x_n\vert \lambda \ge 1$$, since you are near $$x'=0$$).
• 1) By bringing the integral inside of the dot product, you are effectively making $\mathbf{J}(x,\lambda)$ a vector-valued function, correct? But isn't your cutoff function, $a(z)$ a scalar-valued function? Should I just be interpreting $a(z)$ as one scalar component of $\mathbf{J}(x,\lambda)$? If not, then wouldn't this seem to contradict the definition for $\mathbf{J}(x,\lambda)$ you had earlier? – Patch Mar 25 at 6:21
• 2) Assuming we have clarified the issue with $\mathbf{J}(x,\lambda)$ above, lets call $J_k(x, \lambda)$ the $k$-th component of the vector. Then I can see how stationary phase around $(x,z) = (0,0)$ gives us the $(|x_n| \lambda)^{(n-1)/2}$ behavior, but this would only be asymptotic behavior in each coordinate. Moreover, every $J_k$ would satisfy the same big-oh bounds involving the absolute value $|x_n|$ only; so how do I put these all together to get a uniform bound involving the norm, $|x|$? – Patch Mar 25 at 6:21
• @Patch (1) $J$ is indeed vector-valued, but you can handle each coordinate separately. (2) You start with $x$ such that $\vert x\vert \lambda\ge 1$; then you look at the stationary phase method with an integral near a given point $y_0$ of the sphere. You check if you have a stationary point: if not you get a better estimate with $O((\vert x\vert \lambda)^{-N})$ for all $N$. If you land on a stationary point, you check the Hessian. Note that the choice of $y_0$ in the answer above is $e_n$. – Bazin Mar 25 at 9:53