I am reading the book called: "Gauge theory of elementary particle physics" by Ta-Pei Cheng and Ling-Fong Li.
On pages 19-20 they write: (1.78)W[J]=[exp(∫d4x(L1(δδJ))]W0[J], where: W0[J]=∫[dϕ]exp[∫d4x(L0+Jϕ)].
Now, on page 20 they write:" The perturbative expansion in powers of L1 of the exponential in (1.78) gives:
(1.85)W[J]=W0[J]{1+λω1[J]+λ2ω2[J]+…},
where (1.86)ω1[J]=−14!W−10[J]{∫d4x[δδJ(x)]4}W0[J]
ω2[J]=−12(4!)2W−10[J]{∫d4x[δδJ(x)]4}2W0[J]=
=−12(4!)W−10[J]{∫d4x[δδJ(x)]4}ω1[J]
Now, for my question, after I plug ω1[J] into the above last equation I get:
12(4!)2W−10[J]{∫d4x[δδJ(x)]4}W−10[J]{∫d4x[δδJ(x)]4}W0[J]
The last expression is not the same as the above expression, i.e. of −12(4!)2W−10[J]{∫d4x[δδJ(x)]4}2W0[J].
Perhaps instead of ω1[J] it should be −W0[J]ω1[J] in equation (1.86)?
I am puzzled, what do you think?