  We establish inversion formulas of the so called filtered back-projection
type to recover a function supported in the ball in even dimensions from its
spherical means over spheres centered on the boundary of the ball. We also find
several formulas to recover initial data of the form (f,0) (or (0,g)) for the
free space wave equation in even dimensions from the trace of the solution on
the boundary of the ball, provided the initial data has support in the ball.
