  Pion production at threshold from nucleon-deuteron collisions is considered,
with reference to the outgoing channel where the three-nucleon system is bound.
The available experimental data are compared with calculations using accurate
nuclear wavefunctions coming from rigorous solutions of the three-nucleon
quantum mechanical equations. The dominant contributions for pion production
are obtained through matrix elements involving pion-nucleon rescattering
mechanisms in S- and P-waves. S-wave rescattering includes also an isoscalar
contribution which is enhanced because of the off-shell effects. P-wave
rescattering includes also diagrams involving explicitly the Delta degrees of
freedom. It is found that the pion-nucleon S-wave off-shell effects in the
isospin-even channel are of considerable importance. Initial-state interactions
between the proton and the deuteron have in general minor effects on the
spin-averaged and spin-dependent observables, except when the polarization
observable involves interference terms amongst the various helicity amplitudes.
  Keywords: Polarization phenomena. Pion production. Few-nucleon systems.
