## keldysh formalism

kleeorin
הודעות: 236
הצטרף: 15:30 15/05/2007

### keldysh formalism

in Mahan, the expression for G lesser is: $$(1+G^r\Sigma^r )G_0^<(1+\Sigma^aG^a)+G^r\Sigma^< G^a$$ which is not what we talked about before
this means that in order to identify the lesser self energy i also have to know how the retarted (advanced) self energy must look like - which is no difficult in our case but might take time in other cases. is this true?

the second order (which is what im calculatng right now) ill get $$g^<(1+g^r\Sigma^r +g^a\Sigma^a )+g^r\Sigma^< g^a$$ (im almost there). but why is mahan careful with their order? order doesnt matter anymore, right?