Activist investor Carl Icahn purchased $500 million of Apple Inc. shares on Jan. 23 to increase his overall investment in the company to approximately $3.6 billion, according to Reuters.

Icahn confirmed the news on his Twitter account, and has called Apple a "no brainer" on a number of occasions, despite his attempt to get the iPhone company to return more money to its shareholders.

"Even if the story ended with Apple's existing product and software lines, we would still choose to make Apple our largest investment," Icahn said in the letter.

The same day he revealed the purchase, Icahn also filed a letter with the Securities and Exchange Commission, pleading with other shareholders to vote "yea" to his idea for a new $50 billion buyback and stated his case as to why Apple should make more of its $146 billion available.

"Given this massive net cash position and robust earnings generation, Apple is perhaps the most overcapitalized company in corporate history," Icahn said in the letter.

Apple meanwhile has tried telling shareholders to vote down Icahn's idea, saying that it "needed ready access to cash in a fast-evolving and competitive mobile devices industry," according to Reuters.

The iPhone 5S company is in the middle of returning $100 billion to shareholders, including a total share repurchase program of $60 billion. The company has said it's already returned $43 billion in "repurchases and dividends, according to Reuters.

Apple's shares closed up 0.8 percent at $556.18 on Jan. 23.

See Now: OnePlus 6: How Different Will It Be From OnePlus 5?