Joseph Pierre LaSalle (born 28 May 1916 in State College, Pennsylvania; died 7 July 1983 in Little Compton, Rhode Island) was an American mathematician specialising in dynamical systems and responsible for important contributions to stability theory, such as LaSalle's invariance principle which bears his name.
J. P. LaSalle  

Born  Joseph Pierre LaSalle May 28, 1916 State College, Pennsylvania, United States 
Died  July 7, 1983 Little Compton, Rhode Island, United States  (aged 67)
Nationality  American 
Alma mater  California Institute of Technology 
Known for  
Awards 

Scientific career  
Fields  Mathematics, Dynamical Systems, Control theory 
Institutions  University of Notre Dame, RIAS, Brown University 
Doctoral advisor  A.D.Michal^{[3]} 
Joseph LaSalle defended his Ph.D. thesis on ″PseudoNormed Linear Sets over Valued Rings″ at the California Institute of Technology in 1941.^{[3]} In 1946 he joined the Mathematics Department at the University of Notre Dame as an assistant professor and remained there until 1958, becoming a full professor in 1956.^{[4]} During a visit to Princeton in 1947–1948, LaSalle developed a deep interest in differential equations through his interaction with Solomon Lefschetz and Richard Bellman, with whom he developed a close friendship.^{[4]} From 1958 until 1964 LaSalle was based at the Research Institute for Advanced Studies (RIAS) in Baltimore, where he worked closely with Lefschetz and in 1960 published his extension of Lyapunov stability theory,^{[5]} known today as LaSalle's invariance principle.^{[4]}
In 19621963 he was President of the Society for Industrial and Applied Mathematics (SIAM)^{[6]} and was a member of its board of trustees in 1964–1967.^{[4]} In 1964 LaSalle founded the Journal of Differential Equations^{[7]} and served as its EditorinChief until 1980.^{[4]} In 1964 he became the first director of the Center for Dynamical Systems at Brown University, where he was also the chairman of the Division of Applied Mathematics in 1968–1973.^{[8]}
Together with J. K. Hale, LaSalle was the recipient of the 1965 Chauvenet Prize for their article, ″Differential Equations: Linearity vs. Nonlinearity″,^{[9]} published in the SIAM Review.^{[1]} In 1975 he was awarded the Guggenheim Fellowship for applied mathematics.^{[2]}