| CG_smooth | High-dimensional spline smoothing using a matrix-free CG-method. | 
| estimate_trace | Trace estimation of the hat matrix. | 
| generate_test_data | Generate multi-dimensional test data for spline smoothing. | 
| MGCG_smooth | High-dimensional spline smoothing using a matrix-free multigrid preconditioned CG-method. | 
| PCG_smooth | High-dimensional spline smoothing using a matrix-free PCG-method. | 
| predict_smooth | Predictions from model |