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I stumbled across two question that I have no idea on how to answer them.

1) The interaction term in a scalar field theory is [tex]-\frac{\lambda}{4!} \phi^4[/tex]

Why should lambda be positive? (they say look at the energy of the ground state...)

2) Write down the Feynman rules for phi^4

I have no clue as to how you get the two intersecting lines that give is -i lambda vertex. I see where the propagator comes from though.

Any thougths?