years later, Carl David Anderson discovered the positron, for which he received the 1936 Nobel Prize in Physics. In the late 1930s, Oppenheimer became interested in astrophysics, most likely through his friendship with Richard Tolman, resulting in a series of papers. In the first of these, a 1938 paper co-written with Robert Serber titled "On the Stability of Stellar Neutron Cores", Oppenheimer explored the properties of white dwarfs. This was followed by a paper co-written with one of his students, George Volkoff, "On Massive Neutron Cores", in which they demonstrated that there was a limit, the so-called Tolman–Oppenheimer–Volkoff limit, to the mass of stars beyond which they would not remain stable as neutron stars and would undergo gravitational collapse. Finally, in 1939, Oppenheimer and another of his students, Hartland Snyder, produced the paper "On Continued Gravitational Contraction", which predicted the existence of what are today known as black holes. After the Born–Oppenheimer approximation paper, these papers remain his most cited, and were key factors in the rejuvenation of astrophysical research in the United States in the 1950s, mainly by John A. Wheeler. Oppenheimer's papers were considered difficult to understand even by the standards of the abstract topics he was expert in. He was fond of using elegant, if extremely complex, mathematical techniques to demonstrate physical principles, though he was sometimes criticized for making mathematical mistakes, presumably out of haste. "His physics was good", said his student Snyder, "but his arithmetic awful". After World War II, Oppenheimer published only five scientific papers, one of which was in biophysics, and none after 1950. Murray Gell-Mann, a later Nobelist who, as a visiting scientist, worked with him at the Institute for Advanced Study in 1951, offered this opinion: Oppenheimer's diverse interests sometimes interrupted his focus on science. He liked things that were difficult and since much of the scientific work appeared easy for him, he developed an interest in the mystical and the cryptic. In 1933, he learned Sanskrit and met the Indologist Arthur W. Ryder at Berkeley. He eventually read the Bhagavad Gita and the Upanishads in the original Sanskrit, and deeply pondered them. He later cited the Gita as one of the books that most shaped his philosophy of life. His close confidant and colleague, Nobel Prize winner Isidor Rabi, later gave his own interpretation: In spite of this, observers such as Nobel Prize-winning physicist Luis Alvarez have suggested that if