  For each positive $n$, let $u_n = v_n$ denote the identity obtained from the
Adjan identity $(xy) (yx) (xy) (xy) (yx) = (xy) (yx) (yx) (xy) (yx)$ by
substituting $(xy) \rightarrow (x_1 x_2 \dots x_n)$ and $(yx) \rightarrow (x_n
\dots x_2 x_1)$. We show that every monoid which satisfies $u_n = v_n$ for each
positive $n$ and generates the variety containing the bicyclic monoid is
nonfinitely based.
  This implies that the monoid of 2 by 2 upper triangular tropical matrices
over the tropical semiring is nonfinitely based.
