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): where the surface of the -ball centered at with radius , and the usual Euclidean “dot” product; namely and 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 , Next, notice that is rotationally symmetric. Namely, if is a rotation matrix, then where the hermitian adjoint of ; rotation matrices are unitary, so . Changing variables by letting , then, the new domain of integration remains since a rotation, and since , the resulting Jacobian of the transformation is unity. Thus, we find indeed.
From this observation, then, we may assume without loss of generality that is a scalar of the usual basis vector, , namely say , with the Euclidean norm. With this, with . Finally, rewriting this exponential using Euler’s formula and writing the integral using -dim spherical coordinates, we find (identifying ) The iterated product term simplifies using a well-known Gamma-function identity and the right-most integral is identically zero, since the sin terms are symmetric about . Finally, the first integral term is, up to a constant depending only on , , and , equal to the Bessel function (see here, section 3.3). All together, and explicitly writing out this constant, we find from which we arrive finally at a relatively-nice formula Throughout, we tacitly assumed . Indeed, if , the “unit ball” is just the open interval and so the surface of such a ball is the set , so
% If , integrating over (“around”) the circle centered at of radius