Shares of JetBlue Airways Corp. rose sharply after hours on Monday after billionaire activist investor Carl Icahn reported a 9.91% stake in the air carrier — saying its stock was “undervalued and represented an attractive investment opportunity.” That disclosure, made in a regulatory filing on Monday, said that Icahn plans to continue discussions with JetBlue’s management over possible board representation. JetBlue shares JBLU, +2.19% jumped 16.5% in after-hours trading on Monday. The stock is down 28.9 ...