The holmium laser, which operates in a wavelength of 2.1 microns, is highly absorbed by water and has a shallow penetration depth of approximately 0.4 mm. As a pulsed mode laser, the Holmium reduces deeper thermal effects and charring which are more prevalent in Continues Wave lasers (e.g. Thulium, KTP).
In statistical theory, covariance is a measure of how much two random variables change together. In other words, covariance is a measure of the strength of the correlation between two random variables. Also, it can be considered as a generalization of the concept of variance of two random variables.
Note also that if one of the variables has mean 0, then the covariance is simply the expected product. Trivially, covariance is a symmetric operation. \(\cov(X, Y) = \cov(Y, X)\). As the name suggests, covariance generalizes variance. \(\cov(X, X) = \var(X)\). Let \(\mu = \E(X) \). Then \(\cov(X, X) = \E\left[(X - \mu)^2\right] = \var(X) \). Covariance is a linear operation in the first argument, if the second argument is fixed.