Union
$$A \cup B =$$
{ Decision Theory, Logic, Mathematics, Statistics, Systems Theory, Theoretical Computer-Science, context-free, context-sensitive, recursively enumerable, regular }
Symmetric Difference
$$A \ominus B =$$
{ Decision Theory, Logic, Mathematics, Statistics, Systems Theory, Theoretical Computer-Science, context-free, context-sensitive, recursively enumerable, regular }
Difference
$$A - B =$$
{ Decision Theory, Logic, Mathematics, Statistics, Systems Theory, Theoretical Computer-Science }
Cartesian Product
$$A \times B =$$
{ (Decision Theory, context-free),
(Decision Theory, context-sensitive),
(Decision Theory, recursively enumerable),
(Decision Theory, regular),
(Logic, context-free),
(Logic, context-sensitive),
(Logic, recursively enumerable),
(Logic, regular),
(Mathematics, context-free),
(Mathematics, context-sensitive),
(Mathematics, recursively enumerable),
(Mathematics, regular),
(Statistics, context-free),
(Statistics, context-sensitive),
(Statistics, recursively enumerable),
(Statistics, regular),
(Systems Theory, context-free),
(Systems Theory, context-sensitive),
(Systems Theory, recursively enumerable),
(Systems Theory, regular),
(Theoretical Computer-Science, context-free),
(Theoretical Computer-Science, context-sensitive),
(Theoretical Computer-Science, recursively enumerable),
(Theoretical Computer-Science, regular) }
Branches of science