\notniThe symbol \notni is used in mathematical notation to denote that an element is not contained in a set. It is the negation of the \ni symbol, which means 'contains as a member'. This symbol is often used in set theory and logic to express that a particular element does not belong to a specified set.
Element x is not contained in set A
x \notni AThe number 5 is not an element of the set {1, 2, 3, 4}
5 \notni \{1, 2, 3, 4\}Element y is not a member of set B
y \notni B