  A state vector description for relativistic resonances is derived from the
first order pole of the $j$-th partial $S$-matrix at the invariant square mass
value $\sm_R=(m-i\Gamma/2)^2$ in the second sheet of the Riemann energy
surface. To associate a ket, called Gamow vector, to the pole, we use the
generalized eigenvectors of the four-velocity operators in place of the
customary momentum eigenkets of Wigner, and we replace the conventional Hilbert
space assumptions for the in- and out-scattering states with the new hypothesis
that in- and out-states are described by two different Hardy spaces with
complementary analyticity properties. The Gamow vectors have the following
properties:
  - They are simultaneous generalized eigenvectors of the four velocity
operators with real eigenvalues and of the self-adjoint invariant mass operator
$M=(P_\mu P^\mu)^{1/2}$ with complex eigenvalue $\sqrt{\sm_R}$.
  - They have a Breit-Wigner distribution in the invariant square mass variable
$\sm$ and lead to an exactly exponential law for the decay rates and
probabilities.
