The 8 variations were FP at a concert by the Vienna PO in June of 1873 with Brahms conducting. It has become one of his most popular works. In recent years the theme has been attributed to annonymous sources. Not to Haydn at all. Haydn's work has the title 'St. Anthony's Choral'.
Brahms work has the orchestral version as well as one for piano duo.
Brahms Violin and Cello 'Double Concerto' in a Op 102 Brahms was fascinated with his work on this concerto. It was called by many a piece of folly, because a work for violin and cello was very unusual. It was completed in 1887. He worked on this concerto during the summer of 1878 in a Austrian village where he had previously composed the lyrical Second Symphony and where in the following summer he was to compose the G major violin sonata. It was written for a close friend, the famous violinist Joseph Joachim who scored his cadenzas for his performances.