# Universal Algebra

• From the point of view of universal algebra, an algebra (or algebraic structure) is a set $\displaystyle{ A }$ together with a collection of operations on A. An n-ary operation on $\displaystyle{ A }$ is a function that takes $\displaystyle{ n }$ elements of $\displaystyle{ A }$ and returns a single element of A. Thus, a 0-ary operation (or nullary operation) can be represented simply as an element of $\displaystyle{ A }$, or a constant, often denoted by a letter like a. A 1-ary operation (or unary operation) is simply a function from $\displaystyle{ A }$ to $\displaystyle{ A }$, often denoted by a symbol placed in front of its argument, like ~x. A 2-ary operation (or binary operation) is often denoted by a symbol placed between its arguments, like x * y. Operations of higher or unspecified arity are usually denoted by function symbols, with the arguments placed in parentheses and separated by commas, like $\displaystyle{ f }$(x,y,z) or $\displaystyle{ f }$(x1,...,xn). Some researchers allow infinitary operations, such as $\displaystyle{ \textstyle\bigwedge_{\alpha\in J} x_\alpha }$ where $J$ is an infinite index set, thus leading into the algebraic theory of complete lattices. One way of talking about an algebra, then, is by referring to it as an algebra of a certain type $\displaystyle{ \Omega }$, where $\displaystyle{ \Omega }$ is an ordered sequence of natural numbers representing the arity of the operations of the algebra.