Time-asymptotics of a heated string

In the present paper, we study a model of a thermoelastic string that is initially heated. We classify all the possible asymptotic states when time tends to infinity of such a model. Actually, we show that whatever the initial data is, a heated string must converge to a flat, steady string with uniformly distributed heat. The latter distribution is calculated from the energy conservation. In order to obtain the result, we need to take a few steps. In the first two steps, time-independent bounds from above and from below (by a positive constant) of the temperature are obtained. This is done via the Moser-like iteration. The lower bound is obtained via the Moser iteration on the negative part of the logarithm of temperature. In the third step, we obtain a time-independent higher-order estimate, which yields compactness of a sequence of the values of the solution when time tends to infinity. Here, an estimate involving the Fisher information of temperature, together with a recent functional inequality from \cite{CFHS} and an $L^2(L^2)$ estimate of the gradient of entropy, enable us to arrive at a tricky Gr\"{o}nwall type inequality. Finally, in the last steps, we define the dynamical system on a proper functional phase space and study its $\omega$-limit set. To this end, we use, in particular, the quantitative version of the second principle of thermodynamics. Also, the entropy dissipation term and the bound of the entropy from below are useful when identifying the structure of the $\omega$-limit set.