evaluates the nth polynomial of tchebytchev of second kind \( \displaystyle U_n(x) = \frac{\sin(n\arccos x)}{\sin(\arccos x)}\). on \([-1, +1]\).
computes the value of \(T_{n+1}(x)\) knowing the values tn = \(T_n(x)\) and tnm1 = \(T_{n-1}(x)\), This call can be used to create a sequence of values evaluated at the same x and for rising n.