Activist investor Carl Icahn late Monday reported a nearly 10% stake in JetBlue Airways, saying the airline stock is undervalued. Shares of JetBlue gained 21% on Tuesday. Icahn amassed the stake in a series of purchases in January and February, according to regulatory filings. He has had plans to continue discussions with the company "regarding the possibility of board representation," the filings said. JetBlue said in a statement, "We are always open to constructive dialogue with our investors as we continue to execute our plan to enhance value for all of our shareholders and stakeholders."