View source for Annihilator

{{TEX|done}}

''left, of a set $X$ in $R$''

The set $\mathfrak{Z}_l(X)$ of all elements $y$ in $R$ such that $yX = \{0\}$. Here $R$ is a ring or a semi-group (or, generally, a groupoid) with a zero. The right annihilator of a set $X$ in $R$ is defined in a similar manner as the set
$$
\mathfrak{Z}_r(X) = \{ z \in R : Xz = \{0\} \} \ .
$$

The set
$$
\mathfrak{Z}(X) = \mathfrak{Z}_l(X) \cap \mathfrak{Z}_r(X)
$$
is the two-sided annihilator of $X$. In an associative ring (or semi-group) $R$ the left annihilator of an arbitrary set $X$ is a left ideal, and if $X$ is a left ideal of $R$, then $\mathfrak{Z}_l(X)$ is a two-sided ideal of $R$; in the non-associative case these statements are usually not true.

===Modules===
Let $M$ be a left module over a ring $R$, and $X$ a subset of $M$.  The left annihilator of $X$ is   
$$
\mathfrak{Z}_l(X) = \{ z \in R : Xz = \{0\} \} \ .
$$
Again, the left annihilator of an arbitrary set $X$ is a left ideal.

===Linear spaces===
Let $V$ be a [[vector space]] over a field $K$ and $V^*$ the dual space of [[linear functional‎]]s on $V$.  For a subset $X$ of $V$, the annihilator 
$$
X^\circ = \{ f \in V^* : f(X) = \{0\} \} \ .
$$
The annihilator of a general set $X$ is a subspace of $V^*$ and if $\langle X \rangle$ is the subspace of $V$ generated by $X$, then $X^\circ = \langle X \rangle^\circ$.