Quantile Regression, Roger Koenker and Gilbert Bassett Jr., 1978Econometrica, Vol. 46 (The Econometric Society)DOI: 10.2307/1913643 - The seminal paper introducing the concept of quantile regression, defining the pinball loss function.
Custom Objective and Evaluation Metric, XGBoost Developers, 2024 (XGBoost Project) - Provides detailed guidance on implementing custom loss functions for XGBoost, including the required gradient and Hessian returns for non-standard objectives.