Solution
= Solution
The operator $\nabla^{2m}\phi^{2n}$ has <engineering dimension>
$$
2m+2n\frac{d-2}{2}=2m+n(d-2).
$$
The action integral is dimensionless, so
$$
\boxed{[\alpha]=d-2m-n(d-2)=2n-2m-(n-1)d}.
$$
Thus $\alpha$ is a <relevant coupling>, <marginal coupling>, or <irrelevant coupling> according as
$$
\boxed{
d<\frac{2(n-m)}{n-1},\qquad
d=\frac{2(n-m)}{n-1},\qquad
d>\frac{2(n-m)}{n-1}}
$$
respectively.