In the case of IMNX there are two arbitrage selling, there is the selling short of AMGN and buying of IMNX that occurred due to the acquisition (narrowing the spread), and yesterday near the close, what was happening, was arbitrage selling ahead of IMNX being taken out of NDX (since many funds have to buy IMNX if they tracj NDX and the holders of QQQ also have to sell IMNX), since the ratio between IMNX and AMGN is fixed by the offer, a selling drive in IMNX forces arbitrageurs (those that work on the premium difference between acquirer and acquiree) to sell AMGN.
Zeev
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