Any possible path of the particle, going from the initial state to the final state, is approximated as a broken line and included in the measure of the integral ∫ ∫ D q ( t ) = lim N → → ∞ ∞ ( − − i m 2 π π δ δ t ℏ ℏ ) N 2 ( ∏ ∏ j = 1 N − − 1 ∫ ∫ d q j )