$$A^TA$$ (Gramian matrix) Properties:

$$\bf A^TA$$ is a key matrix structure because of the role it plays in orthogonal projection, which is expressed in the form $$A(A^\top A)^{-}A^\top$$ as proven here.

1. Symmetry
2. Positive semidefinite-ness
3. Real and positive eigenvalues
4. The trace is positive (the trace is the sum of eigenvalues)
5. The determinant is positive (the determinant is the product of the eigenvalues)
6. The diagonal entries are all positive
7. Orthogonal eigenvectors
8. Rank of $$A^TA$$ is the same as rank of $$A$$.
9. $$\text{ker}(A^TA)=\text{ker}(A)$$

$$A^TA$$ is the basis for the variance-covariance matrix, which really is the core of regression analysis and PCA - it gives a measure of the relationship of different variables, corresponding to the columns, between and within themselves. So it is important to see how it works.

Let’s pretend that we have a matrix corresponding to returns of different stocks (in the columns) versus 5 consecutive years (in the rows) - completely fictitious stocks and years. Let’s call the matrix, $$A$$:

$A = \begin{bmatrix} & \color{red}{\text{yah(y)}} & \color{blue}{\text{goog(g)}} & \color{green}{\text{ms(m)}} \\ \text{Yr.1} & 1 &8 & 1\\ \text{Yr.2} & -4 &9 & 3 \\ \text{Yr.3} & 5 & 10 & 4 \\ \text{Yr.4} & 7 & 3 & 5\\ \text{Yr.5} & 8 & 7& 6 \end{bmatrix}$

We want to calculate the correlations between the different vectors of returns, one for each company, “packaged” in the matrix $$A$$.

The variance-covariance matrix of the portfolio assuming equal holdings will be:

$$\Large \sigma(A) = \frac{G^TG}{n-1}$$ with $$G$$ being the mean-centered observations and $$n-1$$ corresponding to the number of observations minus $$1$$.

The mean-centered (or demeaned) matrix $$G$$ is:

$\begin{bmatrix} & \color{red}{\text{y}} & \color{blue}{\text{g}} & \color{green}{\text{m}} \\ \text{Yr.1} & -2.4 &0.6 & -2.8\\ \text{Yr.2} & -7.4 &1.6 & -0.8 \\ \text{Yr.3} & 1.6 & 2.6 & 0.2 \\ \text{Yr.4} & 3.6 & -4.4 & 1.2\\ \text{Yr.5} & 4.6 & -0.4& 2.2 \end{bmatrix}$

And the variance-covariance matrix:

$\begin{bmatrix} & \color{red}{y} & \color{blue}{g} & \color{green}{m} \\ \color{red}{y} & 24.30 &-6.70 & 6.85\\ \color{blue}{g} & -6.70 & 7.30 & -2.15 \\ \color{green}{m} & 6.85 & -2.15 & 3.70 \\ \end{bmatrix}$

So it went from the $$5 \times 3$$ $$A$$ matrix to a $$3 \times 3$$ matrix.

The operations involved in calculating the correlation matrix are similar, but the data points are standardized by dividing each one by the standard deviation of the returns of each company (column vectors), right after centering the data points by subtracting the column means as in the covariance matrix:

$\small \text{cor}(A)=\tiny\frac{1}{n-1}\small\begin{bmatrix} \frac{\color{red}{y_1 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{red}{y_2 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{red}{y_3 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{red}{y_4 - \bar{y}}}{\color{red}{sd(y)}} &\frac{\color{red}{y_5 - \bar{y}}}{\color{red}{sd(y)}} \\ \frac{\color{blue}{g_1 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{blue}{g_2 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{blue}{g_3 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{blue}{g_4 - \bar{g}}} {\color{blue}{sd(g)}}& \frac{\color{blue}{g_5 - \bar{g}}}{\color{blue}{sd(g)}}\\ \frac{\color{green}{m_1 - \bar{m}}}{\color{green}{sd(m)}}& \frac{\color{green}{m_2 - \bar{m}}}{\color{green}{sd(m)}} &\frac{\color{green}{m_3 - \bar{m}}}{\color{green}{sd(m)}} & \frac{\color{green}{m_4 - \bar{m}}}{\color{green}{sd(m)}} & \frac{\color{green}{m_5 - \bar{m}}}{\color{green}{sd(m)}}\\ &&\color{purple} {3\times 5 \,\text{matrix}} \end{bmatrix} \begin{bmatrix} \frac{\color{red}{y_1 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{blue}{g_1 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{green}{m_1 - \bar{m}}}{\color{green}{sd(m)}} \\ \frac{\color{red}{y_2 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{blue}{g_2 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{green}{m_2 - \bar{m}}}{\color{green}{sd(m)}} \\ \frac{\color{red}{y_3 - \bar{y}}}{\color{red}{sd(y)}} &\frac{\color{blue}{g_3 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{green}{m_3 - \bar{m}}}{\color{green}{sd(m)}} \\ \frac{\color{red}{y_4 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{blue}{g_4 - \bar{go}}}{\color{blue}{sd(g)}} & \frac{\color{green}{m_4 - \bar{m}}}{\color{green}{sd(m)}} \\ \frac{\color{red}{y_5 - \bar{y}}}{\color{red}{sd(y)}} & \frac{\color{blue}{g_5 - \bar{g}}}{\color{blue}{sd(g)}} & \frac{\color{green}{m_5 - \bar{m}}}{\color{green}{sd(m)}} \\ &\color{purple} {5\times 3 \,\text{matrix}} \end{bmatrix}$

One more quick thing for completeness sake: We have so far a clunky matrix as the result, but in general we want to estimate the portfolio variance: $$1$$ portfolio; $$1$$ variance. To do that we multiply the matrix of variance-covariance of $$A$$ to the left and to the right by the vector containing the proportions or weightings in each stock - $$W$$. Since we want to end up with a scalar single number, it is unsurprising that the algebra will be: $$W^T\,\sigma(A)\,W$$, with the vector of weights (fractions) being in this case $$\color{blue}{3}\times \color{blue}{1}$$ to match perfectly on the left as $$W^T$$, and on the right as $$W$$.

Code in R:

Fictitious data set of returns in billions, percentage (?) - the matrix A:

yah = c(1, - 4, 5, 7, 8)
goog = c(8, 9, 10, 3, 7)
ms = c(1, 3, 4, 5, 6)
returns <- cbind(yah, goog, ms)
row.names(returns) =c("Yr.1","Yr.2","Yr.3","Yr.4", "Yr.5")

Centered matrix (G) of demeaned returns:

demeaned_returns <- scale(returns, scale = F, center = T)

Manual and R function calculation of the variance-covariance matrix:

(var_cov_A = (t(demeaned_returns)%*%demeaned_returns)/(nrow(returns)-1))
cov(returns)   # the R in-built function cov() returns the same results.

Correlation matrix calculation:

We need to divide by the standard deviation column-wise:

demeaned_scaled_returns <- scale(returns, scale = T, center = T)

and then proceed as above:

(corr_A = (t(demeaned_scaled_returns) %*% demeaned_scaled_returns)/(nrow(returns)-1))
cor(returns) # Again, the R function returns the same matrix.