Three ways to solve critical φ⁴ theory on 4-ε dimensional real projective space: perturbation, bootstrap, and Schwinger-Dyson equation

We solve the two-point function of the lowest dimensional scalar operator in the critical $\phi^4$ theory on $4-\epsilon$ dimensional real projective space in three different methods. The first is to use the conventional perturbation theory, and the second is to impose the crosscap bootstrap equation, and the third is to solve the Schwinger-Dyson equation under the assumption of conformal invariance. We find that the three methods lead to mutually consistent results but each has its own advantage.