Dürre, Alexander, Roland Fried, and Daniel Vogel. “The Spatial Sign Covariance Matrix and Its Application for Robust Correlation Estimation”. Austrian Journal of Statistics 46, no. 3-4 (April 12, 2017): 13-22. Accessed September 23, 2020. https://www.ajs.or.at/index.php/ajs/article/view/vol46-3-4-2.