James Simons, a mathematics professor who became one of the most successful hedge fund managers in the world, is retiring as chief executive of his Renaissance Technologies firm on Jan 1., a person briefed in the matter said.

Simons will remain as a nonexecutive chairman, and will keep his money in the funds, the person said. Current Co-Presidents Bob Mercer and Peter Brown will become co-CEOs, the source said.

Renaissance, which manages about $17 billion of assets, declined to comment.

(Reporting by Dan Wilchins)