  In this paper we introduce a new functional invariant of discrete time
dynamical systems -- the so-called t-entropy. The main result is that this
t-entropy is the Legendre dual functional to the logarithm of the spectral
radius of the weighted shift operator on $L^1(X,m)$ generated by the dynamical
system. This result is called the Variational principle and is similar to the
classical variational principle for the topological pressure.
