= Solution
The <model-theoretic algebraic closure> is
$$
\operatorname{acl}(A)=
\{c:\text{$c$ belongs to a finite set definable over $A$}\}.
$$
Equivalently, it consists of elements with finite orbit under $\operatorname{Aut}(\mathcal U/A)$. The <model-theoretic definable closure> is
$$
\operatorname{dcl}(A)=
\{c:\text{$c$ is uniquely definable over $A$}\},
$$
equivalently the set fixed pointwise by every automorphism fixing $A$. Consequently $\operatorname{dcl}(A)\subseteq\operatorname{acl}(A)$.
Back to article page
