| block.true.cov | This function construct a covariance matrix with a block diagonal structure. | 
| est_delta | This function select the optimal thresholding level delta | 
| est_sparseCov | This function computes the thresholding sparse covariance/correlation estimator with the optimal threshold level. | 
| sampleMVN | This function samples MVN based on a given covariance matrix | 
| thresh_op | This function computes the thresholding sparse covariance estimator for a given threshold level. |