I have the following question about arbitrage with options:

Let 0 < K1 < K3 and K2 = (K1+K3)/ 2 be strike prices. Let C1, C2 and C3 be the market prices of

call options with strikes K1, K2 and K3 respectively. What relationship should C1, C2 and C3 satisfy, so that

there do not exist any arbitrage opportunities?

Hint: One can construct a butterfly spread using these three options.