[QUE/SM-04021] SM-PROBLEM

A zipper has two links one ( open) has energy 0 and one (closed) has energy $\Delta$. Label the links as 1,2,...,s,.... The s th link can open only if all the links 1,2,....,(s-1) are open. Assuming the system is in thermal equilibrium at temperature T,  find the canonical partition function. Find the average number of open links  for $T\,\rightarrow\,\infty$