Concerning the Recovery of a Bandlimited Signal or Its Spectrum from a Finite Segment

The relationship between the iterative techniques of Sandberg and Papoulis is clarified. The conditions of Sandberg\´s theorem are not satisfied in general when the problem is that of extrapolating a bandlimited signal outside of a known segment (a problem that is "illposed"). For an interesting special case, the iteration may be applied and the signal recovered exactly.