Unimodular matrix definition? I'm a bit confused. Based on Wikipedia:

In mathematics, a unimodular matrix M is a square integer matrix
  having determinant +1, 0 or −1. Equivalently, it is an integer matrix that is invertible over the integers.

So determinant could be +1, 0 or −1. But a matrix is invertible only if determinat is non-zero! In fact, from Wolfram:

A unimodular matrix is a real square matrix A with determinant det(A) = -1|+1.

Which is right answer?

Well spotted. In a case like this, it's a good idea to check the article's history (using the "View history" link at the top). In the present case, the error was introduced only two days ago by an anonymous user in this edit (which I just reverted).
