Usage:  cheb (n, x)

Returns the value of the nth-order Chebyshev polynomial calculated at
the point x. The Chebyshev polynomials are defined by the equations:

/ cos(n acos(x),    |x| <= 1
Tn(x) = |
\ cosh(n acosh(x),  |x| > 1

If x is a vector, the output is a vector of the same size, where each
element is calculated as y(i) = Tn(x(i)).