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UPDATE: The letter is out and Icahn has lived up to his promise for an interesting open letter.

Activist investor Carl Icahn tweeted a message late Wednesday morning that he would be sending an open letter to Apple Inc. (NASDAQ: AAPL) CEO Tim Cook on Thursday, and that Icahn believes “it will be interesting.” Apple investors found it interesting enough already to push the share price back above $100.

At the same time Icahn also fired off another tweet:

Just over 1yr ago we tweeted our large position in $AAPL & belief the company extremely undervalued. Since then, stock is up 50.6%.

The last time Icahn pushed for a bigger payout for Apple stockholders, the company boosted its share buyback program from $60 billion to $90 billion, increased its dividend and enacted a seven-for-one stock split. All told, Apple said it expects to return more than $130 billion to shareholders by the end of next year.

With about 6 billion shares outstanding and a market cap of $600 billion, Apple only has to listen to Icahn (who holds about 45 million shares) because he can’t be ignored. He is on a roll lately — remember who started the ball rolling on the eBay Inc. (NASDAQ: EBAY) and PayPal split — and whatever he has to say to Cook will surely be “interesting.”

Icahn could, for example, point to Apple’s still massive cash hoard of $164.5 billion — of which more than $133 billion is held offshore — and suggest (insist?) that Apple borrow at next-to-nothing interest rates to spread more of that cash around to shareholders. There is also speculation that Icahn will suggest the Apple buy Nuance Communications Inc. (NASDAQ: NUAN), another company in which Icahn has a 19% stake.

In any event, Apple shares were trading a little lower in the Thursday premarket, down about 0.1% to $100.66, in a 52-week range of $69.31 to $103.74.

ALSO READ: Carl Icahn Says Apple True Value Is $203 — in Over 4,500 Words