Activist investor Carl Icahn has proposed a $US16 billion ($A16.95 billion) share buyback in his latest effort to thwart Dell Inc founder Michael Dell's effort to take the struggling computer maker private. Icahn, now the company's second-largest shareholder after buying 72 million shares from fellow activist investor Southeastern Asset Management Inc, wants the company to buy back up to 1.1 billion Dell shares at $US14 apiece to boost shareholders' return on their investment. The price of the buyback would represent about two-thirds of Dell's current market value of about $US23.5 billion ($A24.90 billion). Dell and other personal computer makers have seen their sales crumble because of the growing popularity of smartphones and tablets. In May, Dell posted a 79 per cent decline in earnings for the most recent quarter.