Work at the Cowles Foundation gave Phelps the chance to meet with other important economists who were working on growth theory , such as David Cass and Nobel Prize winner Tjalling Koopmans . His position at Cowles gave Phelps the chance to interact with Arthur Okun and other notables in the field . He was able to collaborate with other top economists working on growth theory , including David Cass and fellow Nobelist Tjalling Koopmans .