A note on the PQ theorem and the extrapolation of signals

The problem of determining a bandlimited function from its values on a finite interval is ill-conditioned in the sense that although the pertinent inverse map exists, it is discontinuous at every point. Whenever certain closely related general problems are well conditioned in the sense that the inverse operator is continuous, they can be solved using a special case of a known algorithm. In particular, attention is directed to the relation between the PQ theorem, its Hilbert space projection-operator setting, and later work.>