Low-Temperature Expansion of the Free Energy in ASOS Model

We calculate the low-temperature series of the free energy in absolute-value solid-on-solid (ASOS) model to order $u^{23}$ using finite-lattice method. The property of the obtained series and the behavior of their Pad'e approximants confirms us that the roughening transition in ASOS model is of Kosterlitz-Thouless type.