As promised, billionaire investor Carl Icahn sent a seven page letter to Apple shareholders on Thursday, but not before buying an additional $500 million in the company, bringing his total share ownership to $3.6 billion. Icahn’s open letter suggests that shares of Apple are worth $840 each, 52 percent higher than the current trading price. The purpose of the letter, however, is to push Apple to again repurchase shares, something the firm has been reluctant to do.