Thomas J. Fisher Xiaoqian Sun Improved Stein-type shrinkage estimators for the high-dimensional multivariate normal covariance matrix. 1909-1918 2011 55 Computational Statistics & Data Analysis 5 http://dx.doi.org/10.1016/j.csda.2010.12.006 db/journals/csda/csda55.html#FisherS11