Physics 583 (QFT II) Spring 2021 zoom lecture on Tuesday April 6, 2021. Today we begun the discussion of 1/N expansions. We considered the large N limit of 𝜙^4 theory with O(N) symmetry, classified the diagrams and derived the limit N ⟼ ∞. for both the two and the four point functions, relating them to sums over bubble diagrams. We also derived the large N limit in the path integral formulation. In the 2nd half of the lecture we begun the treatment of the O(N) non linear sigma model.