Bartók did not feel comfortable in the USA, and found it very difficult to compose. He was also not very well known in America, and not many people were interested in his music. He and his wife Ditta gave concerts, and for a while, they were given some money to work on a collection of Yugoslav folk songs. They did not have very much money, although he had some friends who gave them enough to live on. Serge Koussevitsky asked him to write the " Concerto for Orchestra ". This soon became Bartók's most popular work, and brought him some money. He was also asked by Yehudi Menuhin to write a "Sonata for Solo Violin". This seemed to make him interested again in composing, and he went on to write his "Piano Concerto No. 3" and began work on his " Viola Concerto".