David L. Elliott

Shauying R. Kou, David L. Elliott, and Tzyh Jong Tarn. Observability of nonlinear systems. Information and Control, 22(1):89-99, February 1973.
Shauying R. Kou, David L. Elliott, and Tzyh Jong Tarn. Exponential observers for nonlinear dynamic systems. Information and Control, 29(3):204-216, November 1975.