For the Schottky–Nordheim barrier, ν (h, F) is given by the particular value ν (f h) of a function ν (ℓ′). The latter is a function of mathematical physics in its own right and has been called the principal Schottky–Nordheim barrier function. An explicit series expansion for ν (ℓ′) is derived in a 2008 paper by J. Deane. The following good simple approximation for ν (f h) has been found: