On some regularity properties of quadratic stochastic processes. Harshinder Singh — 1983 Aequationes mathematicae
On some regularity properties of quadratic stochastic processes. (Short Communication). Harshinder Singh — 1982 Aequationes mathematicae
Exponential-cosine operator-valued functional equation in the strong operator topology. Harshinder Singh; H.L. Vasudeva — 1984 Aequationes mathematicae