1. Princeton Mathematical Series;D. V. Widder,1941
2. I. I. Hirschman, Jr. and D. V. Widder,The convolution transform, Princeton University Press, Princeton, New Jersey, 1954.
3. S. Karlin and W. Studden,Tchebycheff Systems with Applications in analysis and Statistics, Interscience, New-York, to appear in 1966.
4. G. Pólya,On the mean value theorem corresponding to a given linear homogeneous differential equation, Trans. Amer. Math. Soc.24 1922, 312–324.
5. S. Bochner,Harmonic analysis and the theory of probability, University of California Press, Berkeley and Los Angeles, 1955.