Covariance of a nxm matrix with n<m is not positive semidefinite?
87448
Why is a sample covariance matrix singular when sample size is less than number of variables?
68132
Small positive eigenvalues found for a negative definite matrix
27102
Combining two covariance matrices
91409
Semi positive definite matrix is always positive definite matrix
44652
Minimum and Maximum eigenvalue inequality from a positive definite matrix.
87635
Why cannot I obtain a valid SVD of X via eigenvalue decomposition of XX' and X'X?
54211
If $A$ and $B$ are positive definite, then is $B^{-1} - A^{-1}$ positive semidefinite?
88820
Three linked question on non-negative definite matrices.
3408
Do positive semidefinite matrices have to be symmetric?
26386
How to estimate the matrix condition number in the 2-Norm?
112866
Is the matrix filled with the areas of pairwise intersections of disks in a plane always positive semidefinite?
52129
covariance of increasing functions
67820
Showing that $(A_{ij})=\left(\frac1{1+x_i+x_j}\right)$ is positive semidefinite
16779
Why does $\left\| {\left| A \right|} \right\| \le \left\| {\left| I \right|} \right\|$, for every doubly stochastic matrix $A \in M_n$?
18470
A practical way to check if a matrix is positive-definite
91986
What is the agreed upon definition of a "positive definite matrix"?
31631
Proof Verification: Joint variance of the product of a random matrix with a random vector