Activist investor Carl Icahn, who now discloses material information via Twitter, tweeted today that he and Apple CEO Tim Cook will be having dinner together in September to discuss Icahn's desire for Apple to increase its share buyback.

Icahn recently purchased more than $1 billion in Apple stock and believes the company is 'extremely undervalued'.

Spoke to Tim. Planning dinner in September. Tim believes in buyback and is doing one. What will be discussed is magnitude.

Earlier this year, Apple increased its capital return program to more than $100 billion via increased share buybacks and dividends, and issuing bonds for the first time in over a decade.