# Free algebraic system

A free object in a certain class of algebraic systems.

Let be a non-empty class of algebraic systems (see Algebraic systems, class of). A system is called free in the class , or -free, if it belongs to and has a set of generators such that every mapping of into any system from can be extended to a homomorphism . In this case one also says that is free over in the class . A set of generators with this property is called a -free base of the system , and its cardinality is called the rank of . -free systems of the same rank are isomorphic. If the class has a free system of rank , then every system from that admits a generating set of cardinality at most is a homomorphic image of it. A -free base of a -free system is also a minimal generating set; therefore, if the class has isomorphic free systems and of different ranks and , then both cardinal numbers and are finite.

A class of algebraic systems is called trivial or degenerate if in each system the identity is true, that is, if all its systems are one-element systems. Otherwise, the class is called non-trivial or non-degenerate. In any non-degenerate quasi-variety (respectively, variety) of algebraic systems (see Algebraic systems, quasi-variety of; Algebraic systems, variety of) there are free algebraic systems of arbitrary rank. A degenerate class of algebraic systems has only free systems of rank 1.

Suppose that a class has free systems and of finite ranks and , respectively, and suppose that . The isomorphism holds if and only if there are terms

in the signature of such that the following identities are true in :

where ; . If contains a finite system of cardinality , then -free systems of different ranks are not isomorphic. In particular, in all varieties of groups, semi-groups, lattices, and associative rings, free systems of different ranks are not isomorphic. On the other hand, in certain varieties of modules (see Free module) all free modules of finite rank are isomorphic.

There are finitely-presented varieties of algebraic systems (see Algebraic system) in which all free algebraic systems of finite rank are isomorphic. An example is the variety of algebras , of type , defined by the identities

It has been proved (see [3]) that in the varieties of algebras , with -ary operations and -ary operations , defined by the identities

for fixed and , two free algebras of finite rank are isomorphic if and only if

There are universal classes that do not have free systems. An example is the class of groups defined by the universal formulas

(the quantifiers have been omitted). A universal class with free systems of arbitrary finite rank has free systems of arbitrary rank.

Let be a consistent set of universal formulas of the form

where are atomic formulas of the signature . Let

is said to have the -substitution property if for any formula from and for any terms

in variables the following assertion is true:

In a non-degenerate universal class defined by a set of universal formulas there is a free system of finite rank if and only if has the -substitution property [4]. In particular, if all the formulas in are irreducible and contains a positive formula

with disjoint members, then the universal class does not have free systems of rank . For example, the class of totally ordered groups in the signature does not have free algebraic systems of rank .

#### References

 [1] A.I. Mal'tsev, "Algebraic systems" , Springer (1973) (Translated from Russian) [2] B. Jónsson, A. Tarski, "On two properties of free algebras" Math. Scand. , 9 (1961) pp. 95–101 [3] S. Swierczkowski, "On isomorphic free algebras" Fund. Math. , 50 : 1 (1961) pp. 35–44 [4] G. Grätzer, "On the existence of free structures over universal classes" Math. Nachr. , 36 : 3–4 (1968) pp. 135–140