Brunauer et al$.($J$.$ Am$.$ Chem. Soc. $60,309-319,1938)$ derived their

equation for multilayer adsorption by extending Langmuir's theory of

monolayer adsorption to the adsorption of $n$ layers of molecules. The BET isotherm equation is derived by assuming $n=.$ What is the equation that corresponds to a finite value of $n($i$.$e$.,1$