Before revision: This result appeared first in and .
After revision: A special case of this result appeared first in 1963 in a paper by Elmer G. Gilbert, and was later expanded to the current PHB test with contributions by Vasile M. Popov in 1966, Vitold Belevitch in 1968, and Malo Hautus in 1969, who emphasized its applicability in proving results for linear time-invariant systems .