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Let $X_1, \ldots, X_n$ be a random sample from $N_d(\mu, \Sigma)$ and let: $$S=\frac{1}{n}\sum_{i=1}^n(X_i-\bar{X})(X_i-\bar{X})'\,,$$ denote the sample covariance matrix.

If $S$ is invertible, it can be shown that $\bar{X}$ and $S$ are the maximum likelihood estimators for the mean $\mu$ and covariance $\Sigma$ respectively.

What would happen when $S$ is not invertible?

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    $\begingroup$ Then your sample lies on a lower dimensional subspace. You can still do normal inference on the subspace. $\endgroup$ – user603 Dec 13 '15 at 13:46
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In Srivastava, M. S., & von Rosen, D. (2002). Regression models with unknown singular covariance matrix. Linear algebra and its applications, 354(1), 255-273., from Theorem 2.1 we learn:

Assume a $d$-variate normal $\mathbf N (\mathbf \mu, \Sigma)$ with Covariance matrix $\Sigma$ of rank $0<r < d$. Assume an i.i.d. sample of size $n$ and denote the whole sample by the $d \times n$ matrix $\mathbf Y$ (each column is an observation). Let $\mathbf P= I_n - \frac 1n\cdot \mathbf i \mathbf i'$, (where $\mathbf i$ is a column vector of ones). $\mathbf P$ centers a vector around its mean, is idempotent and symmetric. Let $\mathbf S = \mathbf Y\mathbf P \mathbf Y' = ( \mathbf Y\mathbf P)( \mathbf Y\mathbf P)'$. This is the outer product of the de-meaned sample. Denote $\mathbf H$ the $d \times r$ matrix containing the eigenvectors corresponding to the $r$ largest eigenvalues of $\mathbf S$. Let $\mathbf L$ be an $ r \times r$ diagonal matrix containing in its diagonal those $r$ largest eigenvalues. Then :

1) The MLE for the mean-vector $\mathbf \mu$ is the sample mean vector, as usual

2) The MLE for the covariance matrix is

$$\hat \Sigma_{MLE} = \frac 1n \mathbf H \mathbf L \mathbf H'$$

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