1. W. D. Appling,Interval Functions and the Hellinger Integral, Duke Math. J., vol. 29 (1962) pp. 515–520.
2. E. Hellinger,Die Orthogonalinvarianten quadratischer Formen von unendlichvielen Variablen, Diss., Gottingen (1907).
3. W. D. L. Appling,Concerning Nonnegative Valued Interval Functions, Proc. Amer. Math. Soc., vol. 13 (1962), pp. 784–788.