Please answer the following question: Given the following context:  While travelling, Nielsen discovered and then turned against Richard Wagner's music dramas, heard many of Europe's leading orchestras and soloists and sharpened his opinions on both music and the visual arts. Although he revered the music of Bach and Mozart, he remained ambivalent about much 19th-century music. In 1891 he met the composer and pianist Ferruccio Busoni in Leipzig; they were to maintain a correspondence for over thirty years. Shortly after arriving in Paris in early March 1891 Nielsen met the Danish sculptor Anne Marie Brodersen, who was also travelling on a scholarship. They toured Italy together and married in St Mark's English Church, Florence, on 10 May 1891 before returning to Denmark. According to Fanning, their relationship was not only a "love match", but also a "meeting of minds"; Anne Marie was a gifted artist and a "strong-willed and modern-minded woman, determined to forge her own career". This determination would strain the Nielsens' marriage, as Anne Marie would spend months away from home during the 1890s and 1900s, leaving Carl, who was susceptible to opportunities with other ladies, to raise their three young children in addition to composing and fulfilling his duties at the Royal Theatre.Nielsen sublimated his anger and frustration over his marriage in a number of musical works, most notably between 1897 and 1904, a period which he sometimes called his "psychological" period. Fanning writes, "At this time his interest in the driving forces behind human personality crystallized in the opera Saul and David and the Second Symphony (The Four Temperaments) and the cantatas Hymnus amoris and Søvnen". Carl suggested divorce in March 1905 and had considered moving to Germany for a fresh start, but despite several extended periods of separation the Nielsens remained married for the remainder of the composer's life.Nielsen had five children, two of them illegitimate. He had already fathered a son, Carl August Nielsen, in January 1888, before he met Anne Marie. In 1912, an illegitimate...  answer the following question:  What are the last names of the two individuals who were to maintain a correspondence over thirty years?
A:
Nielsen