[QUE/SM-04009] 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 $T\,\rightarrow\,\infty$.