Why do we call $\phi(\partial^2)^n\partial_{\mu_1}\cdots\partial_{\mu_l}\phi$ a double trace operator?
Posted by HChen, at physics.stackexchange.com,
Why do we call $$\phi(\partial^2)^n\partial_{\mu_1}\cdots\partial_{\mu_l}\phi$$ a double trace operator, which appears in…