The call on the 30 strike is trading $2.10 at $2.11, the call on the 32 strike is trading $1.34 - $1.35. Your broker calls you up and says that his market for the 31 call is $1.80 bid at $1.81.
Assuming no transaction fees (other than the bid ask spread) or execution risk , how would you trade the 31 call?
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Since a long butterfly position with equidistant strikes has a non-negative payoff, it follows that the price must always be non-negative.
This means that if we buy the 30 strike once, sell the 31 strike twice, and buy the 32 strike once, this should give us a non-negative number. Checking the math, we obtain
$ 2 . 1 1 − 2 × $ 1 . 8 0 + $ 1 . 3 5 = − $ 0 . 1 4
This means that we should buy the butterfly for 14 cents credit, and choose to wait till expiration to collect the non-negative payoff.
Hence, we should sell the 31 call for $1.80.