UBS has hired a replacement for the maths genius who left for hedge fund Citadel last month, turning to a quantitative trader with two decades under his belt at Citigroup to help it manage trading risk.

Ryan Gould, a managing director at Citi, will succeed Paul Jefferys as the global head of the central risk book at UBS. The CRB is a function that helps the bank manage the risk in its equities trading business and provide liquidity to clients.