This took me a couple of hours, wasted, I am sure many of you already can do this, but I just figured I post it in-case it helps someone with their understanding.
I knew that it is possible to do the bull spread with both puts and calls, I knew they had to workout exactly the same despite different cash flow timeing, however I had to do the math to be convinced. So I took this example.
All options are priced in BS model*
xC1=50 xC2=60 xP1=50 xP2=60 c1=0.2235 c2=0.0015 p1=5.41 p2=14.29 S0=40 rf=10 volatility=10
First method to build the bull spread is using the 50 and 60 calls
Buy the 50 call for 0.2235 and sell the 60 call for 0.0015, total cost is 0.222
assume you borrow the 0.222, you will pay it back in one year as 0.2442
so if ST at expiary is 60, get 10-0.2442=9.7558 and if ST is 50, you pay 0.2442
Second method is
Sell the 60 put for 14.29 and but the 50 put for 5.41, net inflow is 8.88
invest the 8.88 at rf it becomes 9.76 when the option expires
if the stock is at 60 or above you owe nothing to option holder so you end up with 9.76 which is the same thing as in the first method
if the stock is at 50, you will owe 10 but you got 9.76, so you end up paying 0.24 which is the same thing as in first method
there are some slight differences which i assume are due to rounding and the fact the BS model uses continuous rate while I did not when I did the loans, easy to change I just dont feel like it…
Anyway, it is interesting to know that the exact same thing can be done, any comments from experts ?