Update index.md

index.md

@@ -26,6 +26,7 @@
 - [Sampling from lognormal multivariate distribution](https://nbviewer.org/github/luca-fiorito-11/sandy_notebooks/blob/executed_notebooks/notebook_sampling_lognormal.ipynb)
 - [Normal vs lognormal sampling: the U-235 case](https://nbviewer.org/github/luca-fiorito-11/sandy_notebooks/blob/executed_notebooks_v1.1/notebook_lognorm_sampling_u5.ipynb)
 - [How lognormal samples react to large uncertainties](https://nbviewer.org/github/luca-fiorito-11/sandy_notebooks/blob/executed_notebooks_v1.1/notebook_check_log_sampling_magnitude.ipynb)
+- [Negative eigenvalues: U-235 xs covariance matrix from ENDF/B-VIII.0](https://nbviewer.org/github/luca-fiorito-11/sandy_notebooks/blob/executed_notebooks_v1.1/notebook_u235_endfb80_eigenvalues.ipynb)
 
 ### Consistency checks
 - [Perturbations to redundant cross section are consistently distributed to partial components](https://nbviewer.org/github/luca-fiorito-11/sandy_notebooks/blob/executed_notebooks_v1.1/notebook_pert_mf4_redundant.ipynb)