The current issue of AJP has an article purporting to show an elementary
proof that the Boltzmann distribution function is an exponential. However,
it omits the key step, namely a proof of the fact that the only real
continuous function f that satisfies:
f(x+y) = f(x)f(y)
for independent variables x and y is an exponential. Instead they make
reference to a graduate level math text that has a rather sophisticated
proof. I have attempted to construct a simpler proof of this key step here: