By Edwin Chan SAN FRANCISCO (Reuters) – Activist investor Carl Icahn picked up another $500 million of Apple Inc shares on Thursday to take his investment in the iPhone maker to $3.6 billion, while ratcheting up his months-long campaign to pry open the companys cash hoard. Icahn, who repeatedly has called Apple a no brainer even as he wages a campaign to get it to return more cash to shareholders, tweeted his latest move just a day after revealing a position of over $3 billion in the company. In a lengthy letter to shareholders filed with the Securities and Exchange Commission on Thursday, Icahn urged shareholders to vote yea to his proposal for a new $50 billion buyback, and laid out familiar arguments as to why Apple should share more of its $146 billion cash hoard. Apples strong market position in smartphones and tablets does not justify the companys official stance that it needed to maintain reserves to compete in a fast-evolving consumer electronics industry, Icahn said.