In Rudin's Real and Complex Analysis, there is a symbol for stating measure
$\lambda$ is absolutely continuous with respect to measure
$\mu$. The symbol is like
$\lambda\ll\mu$, but not exactly.
If one doesn't want to load the whole lot of mathabx, that changes the shape of many symbols, it's possible to load only the one needed, see Importing a single symbol font from a different font.
The definition would be
using the unicode-math name or, if the alternative way by pisymbol is preferred,
The complete series of declarations in the preamble should be
If your TeX distribution is fairly recent you can leave out the commented lines (starting with
With the alternative method that doesn't reserve a math alphabet, the code to write in the preamble is
Have a look at “How to look up a math symbol?” for ideas how you can easily find a particular symbol.
The symbols list has
Unicode defines the symbols as