Activist investor Carl Icahn on Tuesday announced on Twitter that his firm holds a large stake in Apple. “We currently have a large position in Apple. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come,” Icahn posted on his Twitter account, adding that he believes Apple should buy back more shares than it is currently planning to repurchase. Following the news, Bloomberg reports that Icahn’s stake in Apple is worth more than $1 billion and he has built his position over the last month alone. According to Bloomberg, Icahn sees Apple shares climbing back into the $600 range. Apple’s stock climbed more than 4% following Icahn’s post on Twitter.