Closed $1\mathrm{/}2$-Elasticae in the Hyperbolic Plane

We study critical trajectories in the hyperbolic plane for the $1\mathrm{/}2$-Bernoulli’s bending energy with length constraint. Critical trajectories with periodic curvature are classified into three different types according to the causal character of their momentum. We prove that closed trajectories arise only when the momentum is a time-like vector. Indeed, for suitable values of the Lagrange multiplier encoding the conservation of the length during the variation, we show the existence of countably many closed trajectories with time-like momentum, which depend on a pair of relatively prime natural numbers.