d,  see  <a href="weyl.html#Chevie.Weyl"><code>Weyl</code></a>; the corresponding finite groups play a deep role in several areas of mathematics.</p><p>Coxeter  groups  have  a  nice  solution  to the word problem. The <em>length</em> <code>l(w)</code>  of an element  <code>w‚àà W</code> is  the minimum number  of elements of <code>S</code> of which it is a product (since the elements of <code>S</code> are involutions, we do not need inverses). An expression of <code>w</code> of minimum length is called