TY  - JOUR
AU  - G. J. Butler
TI  - The existence of periodic solutions of an autonomous second order non-linear differential equation
JO  - Annales Polonici Mathematici
PY  - 1974
SP  - 165
EP  - 174
VL  - 30
IS  - 2
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/articles/10.4064/ap-30-2-165-174/
DO  - 10.4064/ap-30-2-165-174
LA  - en
ID  - 10_4064_ap_30_2_165_174
ER  -