Dear Prof Thijssen, Thank you so very much for the video! On calculating J* using the cumulant expansion at 37:52, I was wondered why we do not need to consider ln(Z_0) according to the truncated expansion H'=ln(Z_0)+? Please, could you point me out. Thank you. Very kind regards, Z. W.
It does not necessarily give a very good result, but the quality of the approximation depends on the number of terms you take into account. You can obtain very good results, provided you go through the effort of enumerating and calculating the higher order diagrams.
Dear Prof Thijssen,
Thank you so very much for the video!
On calculating J* using the cumulant expansion at 37:52, I was wondered why we do not need to consider ln(Z_0) according to the truncated expansion H'=ln(Z_0)+?
Please, could you point me out.
Thank you.
Very kind regards,
Z. W.
Oh I got it. The constant A=ln(Z_0) cancels when combining equation for J' and equation for -J' in order to calculate J*.
Sorry for my late reaction. You got it right!
Dose the cumulant expansion always lead to a better result and why? Thank you.
It does not necessarily give a very good result, but the quality of the approximation depends on the number of terms you take into account. You can obtain very good results, provided you go through the effort of enumerating and calculating the higher order diagrams.