The asymptotics of the gap in the Mathieu equation

We provide a simple proof that the kth gap, Δ_k, for the Mathieu operator −d^2dx^2 + 2κcos(2x) is Δ_k = 8(κ4)^k[(k − 1)!]^(−2)(1 + o(k^(−2))), a result obtained (up to the value of an integral) by Harrell. The key observation is that what is involved is tunneling in momentum space.