Then we see that the ℏ ℏ / c factor is very small so for k not too large, we expand 1 + x 2 ≈ ≈ 1 + x 2 / 2 , and multiply: ω ω ( k ) ≈ ≈ m 0 c 2 ℏ ℏ + ℏ ℏ k 2 2 m 0 . This gives the non-relativistic approximation discussed above. If we start with the non-relativistic Schrödinger equation we will end up without the first, rest mass, term.