View source for Free algebra

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''in a class $\mathfrak K$ of universal algebras''

An algebra $F$ in $\mathfrak K$ with a free generating system (or base) $X$, that is, with a set $X$ of generators such that every mapping of $X$ into any algebra $A$ from $\mathfrak K$ can be extended to a homomorphism of $F$ into $A$ (see [[Free algebraic system|Free algebraic system]]). Any non-empty class of algebras that is closed under subalgebras and direct products and that contains non-singleton algebras, has free algebras. In particular, free algebras always exist in non-trivial varieties and quasi-varieties of universal algebras (see [[Variety of universal algebras|Variety of universal algebras]]; [[Algebraic systems, quasi-variety of|Algebraic systems, quasi-variety of]]). A free algebra in the class of all algebras of a given signature $\Lambda$ is called absolutely free. An algebra $A$ of signature $\Lambda$ is a free algebra in some class of universal algebras of signature $\Lambda$ if and only if $A$ is intrinsically free, that is, if it has a generating set $X$ such that every mapping of $X$ into $A$ can be extended to an endomorphism of $A$. If a free algebra has an infinite base, then all its bases have the same cardinality (see [[Free Abelian group|Free Abelian group]]; [[Free algebra over a ring|Free algebra over a ring]]; [[Free associative algebra|Free associative algebra]]; [[Free Boolean algebra|Free Boolean algebra]]; [[Free group|Free group]]; [[Free semi-group|Free semi-group]]; [[Free lattice|Free lattice]]; [[Free groupoid|Free groupoid]]; [[Free module|Free module]]; and also [[Free product|Free product]]). Clearly, every element of a free algebra with a base $X$ can be written as a word over the alphabet $X$ in the signature of the class being considered. It is natural to ask: When are different words equal as elements of the free algebra? In certain cases the answer is almost trivial (semi-groups, rings, groups, associative algebras), while in others it is fairly complicated (Lie algebras, lattices, Boolean algebras), and sometimes it does not have a recursive solution (alternative rings).



====Comments====
Sometimes the phrases  "free algebra"  and  "free algebraic system"  are identified in meaning. See [[Free algebraic system|Free algebraic system]].