One line to run them all: SuperEasy massive neutrino linear response in ????-body simulations

We present in this work a novel and yet extremely simple method for incorporating the effects of massive neutrinos cosmological $N$-body simulations. This so-called "SuperEasy linear response" approach is based upon analytical solutions to collisionless Boltzmann equation clustering free-streaming limits, which are then connected by rational function interpolation with cosmology-dependent coefficients given algebraic expressions model parameters. The outcome {\it one-line modification} gravitational potential that requires only cold matter density contrast as real-time input, can be incorporated into any code Particle--Mesh component no additional implementation cost. To demonstrate its power, we implement SuperEasy publicly available \gadgetcode{} code, show neutrino mass sums not exceeding $\sum m_\nu \simeq 1$ eV, total power spectra sub-1\% sub-0.1\% agreement those from state-of-the-art response simulations literature. Aside minimal cost, compared existing simulation methods, has better memory efficiency, incurs runtime overhead relative standard $\Lambda$CDM simulation, post-processing. nature allows limited computational resources diverted modelling other physical interest, e.g., baryonic physics via hydrodynamics.