Nielsen's sonata Op. 35 was composed in 1912 for a concert which was planned to take place the following year. According to Nielsen's diary, he must have started composing before summer 1912. The pianist at the planned concert, Henrik Knudsen, followed the compositional process closely, and according to the correspondence he had Nielsen's draft at his disposal for some periods. The work was completed in September 1912. See more about this work at The Royal Library