Period Doublings in Coupled Parametrically Forced Damped Pendulums
  We study period doublings in $N$ $(N=2,3,4, \dots)$ coupled parametrically forced damped pendulums by varying $A$ (the amplitude of the external driving force) and $c$ (the strength of coupling). With increasing $A$, the stationary point undergoes multiple period-doubling transitions to chaos. We first investigate the two-coupled case with $N=2$. For each period-doubling transition to chaos, the critical set consists of an infinity of critical line segments and the zero-coupling critical point lying on the line $A=A^*_i$ in the $A-c$ plane, where $A^*_i$ is the $i$th transition point for the uncoupled case. We find three kinds of critical behaviors, depending on the position on the critical set. They are the same as those for the coupled one-dimensional maps. Finally, the results of the $N=2$ case are extended to many-coupled cases with $N \geq 3$, in which the critical behaviors depend on the range of coupling. 
Sang-Yoon Kim (Georgia Institute of Technology and Kangwon National   University) and Kijin Lee (Kangwon National University)