While teaching , MacMahon continued his studies and completed his PhD at Columbia in 1923 .
During this period , MacMahon also befriended the artist Georgia O'Keeffe and helped to introduce her to liberal political ideas .
While teaching , MacMahon continued MacMahon 's studies and completed his PhD at Columbia in 1923 .
During this period , MacMahon also befriended the artist Georgia O'Keeffe and helped to introduce her to liberal political ideas .