In mathematics, and in particular universal algebra, the concept of n-ary group (also called n-group or multiary group) is a generalization of the concept of group to a set G with an n-ary operation instead of a binary operation. By an n-ary operation is meant any set map f: Gn → G from the n-th Cartesian power of G to G. The axioms for an n-ary group are defined in such a way that they reduce to those of a group in the case n = 2. The earliest work on these structures was done in 1904 by Kasner and in 1928 by Dörnte; the first systematic account of (what were then called) polyadic groups was given in 1940 by Emil Leon Post in a famous 143-page paper in the Transactions of the American Mathematical Society.
The easiest axiom to generalize is the associative law. Ternary associativity is the polynomial identity (abc)de = a(bcd)e = ab(cde), i.e. the equality of the three possible bracketings of the string abcde in which any three consecutive symbols are bracketed. (Here it is understood that the equations hold for arbitrary choices of elements a,b,c,d,e in G.) In general, n-ary associativity is the equality of the n possible bracketings of a string consisting of n+(n-1) = 2n-1 distinct symbols with any n consecutive symbols bracketed. A set G which is closed under an associative n-ary operation is called an n-ary semigroup. A set G which is closed under any (not necessarily associative) n-ary operation is called an n-ary groupoid.
Inverses / unique solutions
The inverse axiom is generalized as follows: in the case of binary operations the existence of an inverse means ax = b has a unique solution for x, and likewise xa = b has a unique solution. In the ternary case we generalize this to abx = c, axb = c and xab = c each having unique solutions, and the n-ary case follows a similar pattern of existence of unique solutions and we get an n-ary quasigroup.
Definition of n-ary group
An n-ary group is an n-ary semigroup which is also an n-ary quasigroup.
Identity / neutral elements
In the 2-ary case, i.e. for an ordinary group, the existence of an identity element is a consequence of the associativity and inverse axioms, however in n-ary groups for n ≥ 3 there can be zero, one, or many identity elements.
An n-ary groupoid (G, ƒ) with ƒ = (x1 ◦ x2 ◦ . . . ◦ xn), where (G, ◦) is a group is called reducible or derived from the group (G, ◦). In 1928 Dörnte  published the first main results: An n-ary groupoid which is reducible is an n-ary group, however for all n > 2 there exist n-ary groups which are not reducible. In some n-ary groups there exists an element e (called an n-ary identity or neutral element) such that any string of n-elements consisting of all e's, apart from one place, is mapped to the element at that place. E.g., in a quaternary group with identity e, eeae = a for every a.
An n-ary group containing a neutral element is reducible. Thus, an n-ary group that is not reducible does not contain such elements. There exist n-ary groups with more than one neutral element. If the set of all neutral elements of an n-ary group is non-empty it forms an n-ary subgroup.
Some authors include an identity in the definition of an n-ary group but as mentioned above such n-ary operations are just repeated binary operations. Groups with intrinsically n-ary operations do not have an identity element.
The axioms of associativity and unique solutions in the definition of an n-ary group are stronger than they need to be. Under the assumption of n-ary associativity it suffices to postulate the existence of the solution of equations with the unknown at the start or end of the string, or at one place other than the ends; e.g., in the 6-ary case, xabcde=f and abcdex=f, or an expression like abxcde=f. Then it can be proved that the equation has a unique solution for x in any place in the string. The associativity axiom can also be given in a weaker form - see page 17 of "On some old and new problems in n-ary groups".
The following is an example of a three element ternary group, one of four such groups
- Dudek, W.A. (2001), "On some old and new problems in n-ary groups", Quasigroups and Related Systems, 8: 15–36.
- W. Dörnte, Untersuchungen über einen verallgemeinerten Gruppenbegriff, Mathematische Zeitschrift, vol. 29 (1928), pp. 1-19.
- E. L. Post, Polyadic groups, Transactions of the American Mathematical Society 48 (1940), 208–350.
- Wiesław A. Dudek, Remarks to Głazek's results on n-ary groups, Discussiones Mathematicae. General Algebra and Applications 27 (2007), 199–233.
- Wiesław A. Dudek and Kazimierz Głazek, Around the Hosszú-Gluskin theorem for n-ary groups, Discrete Mathematics 308 (2008), 486–4876.
- S. A. Rusakov: Some applications of n-ary group theory, (Russian), Belaruskaya navuka, Minsk 1998.