The symbol kind of looks like this: ε, but it's more like a sideways u with a line through the middle.
[Math] What does the symbol described below mean? (looks like: sideway u with a line through the middle)
notation
Related Solutions
It is the interior product (Wikipedia page, MathWorld page). It's defined on page 334-336 of Lee's book:
Quoting Mariano Suárez-Alvarez:
The symbol denotes whatever the author tells you it will denote in his comments about notation, and there is a special place in hell for users of unexplained notation.
Best Answer
Do you mean $\in$? This means "an element of". For example, if we denote the set of natural numbers by ${\mathbb N}$ then $1 \in {\mathbb N}$. Similarly, $1,2,3, \ldots \in {\mathbb N}$, and $ - 1 \notin {\mathbb N}$. Sometimes you might also see $\ni$, which some authors use for "such that". You might also be referring to $\epsilon$, which is the same as $\varepsilon $, or perhaps you mean $\not\subset$, which usually means "not a subset of".