This approach originates from the assumption that the state of the BEC can be described by the unique wavefunction of the condensate ψ ψ ( r → → ) . For a system of this nature, | ψ ψ ( r → → ) | 2 is interpreted as the particle density, so the total number of atoms is N = ∫ ∫ d r → → | ψ ψ ( r → → ) | 2