TY  - BOOK
AU  - Lefschetz, S, 
AU  - 
TI  - Contributions to the theory of nonlinear oscillations, Vol.1
T2  - Annals of Mathematics Studies, Vol.020
U1  - R  AMS.020 
PY  - 1950///
CY  - Princeton University Press, New Jersey
KW  - Annals of Mathematics Studies
ER  -