Estimate the density quantile function
density_quantile_function(y, x, u, alpha, sparsity, bandwidth_type)
Vector of dependent data
Matrix of covariates
Quantile residuals
Probability level
iid or ind
Bofinger, Chamberlain or Hall-Sheather
For the iid and nid method, see Koenker (1994), and Hendricks and Koenker (1992). For the bandwidth types, see Bofinger (1975), Chamberlain (1994), and Hall and Sheather(1988).