  We apply the Keldysh formalism in order to derive a current formula easy to
use for a system with many sites, one of which is interacting. The main
technical challenge is to deal with the lesser Green function. It turns out
that, in the case of the left-right symmetry, the knowledge of the lesser Green
function is not necessary and an exact current formula can be expressed in
terms of retarded Green functions only. The application is done for a
triangular interferometer which gives a good account of the Fano-Kondo effect.
It is found that the interference effects, in the context of Kondo
correlations, give rise to a point in the parameters space where the
conductance is temperature-independent. We include a comparison with the
results from the Ng's ansatz, which are less accurate, but can be used also in
the absence of the above mentioned symmetry.
