The derivative of the bi-square psi function.

If abs(u) > k , psi_prime(u) = 0. Else, psi_prime(u) = (1 - (u/k)^2)^2 - (2u/k)^2 (1 - (u/k)^2).

Usage

psi_prime(u, k = 1.96)

Arguments

u

Can be a single value, vector, or matrix.

k

The tuning parameter. Can be a scalar or the same dimension as u.

Value

The bi-square psi-prime function.