Solution
= Solution
\b[The <formal language> is a <regular language>.] A <regular expression> for it is
$$
\boxed{(ab)^*(bc)^*.}
$$
The first <Kleene star> independently chooses $m$ copies of the <word> $ab$, and the second independently chooses $n$ copies of $bc$. Their <concatenation> produces exactly the required <words>. Either count may be zero, including both counts zero, which gives the <empty word>. By <Kleene theorem>, the <regular expression> establishes regularity.