  We write down a minimal basis for dimension-six gauge-invariant four-fermion
operators, with some operator replacements with respect to previous ones which
make it simpler for calculations. Using this basis we classify all four-fermion
operator contributions involving one or two top quarks. Taking into account the
different fermion chiralities, possible colour contractions and independent
flavour combinations, a total number of 572 gauge-invariant operators are
involved. We apply this to calculate all three-body top decay widths t -> d_k
u_i dbar_j, t -> d_k e_i+ nu_j, t -> u_k u_i ubar_j, t -> u_k e_j+ e_i-, t ->
u_k nu_j nu_i (with i,j,k generation indices) mediated by dimension-six
four-fermion operators, including the interference with the Standard Model
amplitudes when present. All single top production cross sections in pp, p pbar
and e+ e- collisions are calculated as well, namely u_i d_k -> d_j t, dbar_j
d_k -> ubar_i t, u_i dbar_j -> dbar_k t, u_i u_k -> u_j t, u_i ubar_j -> ubar_k
t, e+ e- -> ubar_k t and the charge conjugate processes. We also compute all
top pair production cross sections, ubar_i u_j -> t tbar, dbar_i d_j -> t tbar,
u_i u_j -> t t and e+ e- -> t tbar. Our results are completely general, without
assuming any particular relation among effective operator coefficients.
