N(t) = a0 + a1N(t-1) + a2N(t-2) + ... + adN(t-d) + e(t )
against the alternative:
Nt = F(N(t-1), N(t-2), ..., N(t-d)) + e(t)
This is the Tukey one-degree-of-freedom test of linearity developed by Tsay
(1986). Orders up to 5 is permissible. [although the code is easily
extended].