Original question:
The symbol looks like a numeral 1 written like an R in $\mathbb{R}$. It has a double vertical line and a serif at the bottom. It represents a function of a parameter: $1\_{\{0,1\}}(x)$. Adding it as a factor to your formula limits your expression to a specific set or range of x values. In my example, if x is neither 0 nor 1, the whole expression is zero. Like a security switch against over- or underflow of your parameters.


What is it called, please? (I want to learn more about it, but I cannot talk about it without a word for it).


Answer:
Thank you! Now I have two names for it:


* Characteristic Function and
* Indicator Function


In English, an indicator function is a characteristic function, so I will use the word indicator function from now on, as it is the specific usage I was looking for.


To type it in TeX, thanks, I have learned to add package bbold and then mathbb{} the number 1. So that marginal hitch is history, too. 



Looking for the name of a mathematical symbol that looks remotely like 1 (answer: indicator function)
The function $\mathbb 1\_A$, whose values are $1$ for arguments in the set $A$ and $0$ for arguments outside $A$, is usually called the characteristic function of the set $A$. 


Unfortunately, the same terminology is also used with other meanings. For example, in probability theory, "characteristic function" often means the Fourier transform of a probability distribution.


