In 1927, E. Schrödinger proposed his famous wave equation,
i h dΨ/dt)partial=h^2 . Nabla^2 Ψ/2m + V Ψ . . . . (1)
Schrödinger over complicated QM via equation 1 which describes the square root of the energy density (rather than the energy density itself).
One of the simple ways to solve SE is to solve its square (Ψ^2=Ψ.Ψ*) like the heat equation for example (T), then find its square root which would describe Ψ itself.