2. THE EFFECTIVE ACTION 33
Therefore the effective interaction form of the renormalization group equa-
tion (RGE) is
I[Λ](φ + a) .
Note that in this expression the field a no longer has to be low-energy.
We obtain a variant of the renormalization group equation by considering
effective interactions I[Λ] which are functionals of all fields, not just low-
energy fields, and using the equation above. This equation is invertible; it
is valid even if Λ Λ.
We will always deal with this invertible effective interaction form of the
RGE. Henceforth, it will simply be referred to as the RGE.
2.2. We will often deal with integrals of the form
exp (Φ(x)/ + I(x + a)/ )
over a vector space U, where Φ is a quadratic form (normally negative
definite) on U. We will use the convention that the “measure” on U will be
the Lebesgue measure normalised so that
exp (Φ(x)/ ) = 1.
Thus, the measure depends on .
2.3. Normally, in quantum field theory textbooks, one starts with an
S(φ) = −
where the interacting term I(φ) is a local action functional. This means that
it can be written as a sum
D1,j(a) · · · Dk,j(a)
and the Di,j are differential operators on M. We also require that I(φ) is
at least cubic modulo .
As I mentioned before, the local interaction I is supposed to be thought
of as the scale ∞ effective interaction. Then the effective interaction at scale
Λ is obtained by applying the renormalization group flow from energy ∞
down to energy Λ. This is expressed in the functional integral
I[Λ](a) = log
I(φ + a) .