concatenation

let $\text{[math]}$ and $\text{[math]}$ be strings. then $\text{[math]}$ denotes the concatenation of $\text{[math]}$ and $\text{[math]}$ , that is, the string formed by making a copy of $\text{[math]}$ and following it by a copy of $\text{[math]}$ .
more precisely, if $\text{[math]}$ is the string composed of $\text{[math]}$ symbols $\text{[math]}$ and y is the string composed of $\text{[math]}$ symbols $\text{[math]}$ , then $\text{[math]}$ is the string of length $\text{[math]}$ .
[cite:@john_automata_2006]