Apple’s Carl Icahn effect, redux by Philip Elmer-DeWitt @FortuneMagazine October 1, 2013, 2:59 PM EDT E-mail Tweet Facebook Google Plus Linkedin Share icons FORTUNE — He’s done it again. In August, corporate raider Carl Icahn announced on Twitter that he believed Apple AAPL at $470 was “extremely undervalued.” He said that he had taken a “large position” in the company, had talked to Tim Cook about accelerating the company’s record $60-billion stock buyback program, and would be meeting with Cook to discuss it further. On the strength of two tweets from Icahn, the stock soared and the company gained $17 billion in market value in just over an hour. Icahn issued another tweet Tuesday morning at 10:24 a.m. Eastern: @Carl_C_Icahn: Had a cordial dinner with Tim last night. We pushed hard for a 150 billion buyback. We decided to continue dialogue in about three weeks. The effect on Apple’s share price was nearly instantaneous.