Hedge fund D.E. Shaw bought 2.36 million shares, while Adage Capital and Renaissance Capital bought 1.91 million shares and 1.21 million shares of the stock.Towards the end of the third quarter, Berkshire Hathaway announced it would buy back shares. Buffett, who had often been critical of buybacks, was sending a strong signal that he believed the shares of the holding company were undervalued. Berkshire Hathaway's own 13F filings are often eagerly anticipated as investors look for clues as to what the Wizard of Omaha is buying. Buffett surprised investors with a fresh stake in IBM (IBM), foraying into the technology space which he had long avoided. Its biggest acquisition in 2011 was chemical company Lubrizol for $9 billion. Shares of Berkshire Hathaway are down 7% so far in 2011. The stock has two outperform ratings and one hold rating.