Louis Meunier

Remarks

A Special Spherical Integral

I recently had to deal with the following integral (in computing the Fourier transform of the surface measure of a sphere): I=B(x,t)eizydy, where B(x,t)Rd the surface of the d-ball centered at xRd with radius t>0, and zy the usual Euclidean “dot” product; namely x,t, and z are all fixed. Finding a “nice” closed form for the expression proceeds as follows. First, we translate so the integral is about the origin, by sending yyx, I=B(0,t)eizyeizxdy=eizxB(0,t)eizydy=:eizxI(z). Next, notice that I(z) is rotationally symmetric. Namely, if A:RdRd is a rotation matrix, then I(Az)=B(0,t)exp(i(Az)y)dy=B(0,t)exp(izAy)dy, where A the hermitian adjoint of A; rotation matrices are unitary, so A=A1. Changing variables by letting u:=A1y, then, the new domain of integration remains B(0,t) since A a rotation, and since det(A1)=det(A)1=1, the resulting Jacobian of the transformation is unity. Thus, we find I(Az)=I(z) indeed.

From this observation, then, we may assume without loss of generality that z is a scalar of the usual basis vector, e1, namely say z=|z|e1, with || the Euclidean norm. With this, I(z)=B(0,t)exp(i|z|y1)dy, with y=(y1,,yd). Finally, rewriting this exponential using Euler’s formula and writing the integral using d-dim spherical coordinates, we find (identifying y1=tcosϕ1) I(z)=0π02π[cos(|z|tcos(ϕ1))+isin(|z|tcos(ϕ1))]×td1sind2(ϕ1)sin(ϕd2)dϕ1dϕd1=2πtd1[j=1d30πsinj(ϕ)dϕ]×[0πcos(|z|tcosϕ)sind2(ϕ)dϕ+0πisin(|z|tcosϕ)sind2(ϕ)dϕ]. The iterated product term simplifies using a well-known Gamma-function identity j=1d30πsinj(ϕ)dϕ=π(d3)/2j=1d3Γ(j+12)Γ(j+22)=π(d3)/2Γ(d12), and the right-most integral is identically zero, since the sin terms are symmetric about π/2. Finally, the first integral term is, up to a constant depending only on d, t, and |z|, equal to the Bessel function Jd22(t|z|) (see here, section 3.3). All together, and explicitly writing out this constant, we find I(z)=2πtd1π(d3)/2Γ(d12)Γ(12)Γ(d12)(12t|z|)(d2)/2Jd22(t|z|)=(2π)d/2td/2|z|d22Jd22(t|z|), from which we arrive finally at a relatively-nice formula I=(2π)d/2td/2|z|(d2)/2eizxJd22(t|z|). Throughout, we tacitly assumed d>2. Indeed, if d=1, the “unit ball” is just the open interval (xt,x+t) and so the surface of such a ball is the set xt,x+t, so I=eizy|y=xtx+t=eiz(x+t)eiz(xt)=eizx[eizteizt]=2ieizxsin(x)=2icos(xz)sin(x)2sin(x)sin(xz).

% If d=2, integrating over (“around”) the circle centered at x of radius t