  Given a real valued function on R^n we study the problem of recovering the
function from its spherical means over spheres centered on a hyperplane. An old
paper of Bukhgeim and Kardakov derived an inversion formula for the odd n case
with great simplicity and economy. We apply their method to derive an inversion
formula for the even n case. A feature of our inversion formula, for the even n
case, is that it does not require the Fourier transform of the mean values or
the use of the Hilbert transform, unlike the previously known inversion
formulas for the even n case. Along the way, we extend the isometry identity of
Bukhgeim and Kardakov for odd n, for solutions of the wave equation, to the
even n case.
