Let K₁ < K₂ be two strike prices of two European calls both with the same expiration date, T.

Prove:

The difference between the premiums of two European calls on the same underlying asset for the same expiration date but with two different strike prices, cannot exceed the difference between the present value of the difference between the strike prices. In notation, prove that:

c₁ - c₂ (K₂ - K₁)e^{-r(T - t)} .