Jahn 1856, p. 315: "Auch dutzten sie sich ... – das war damals bei solchen Altersunterschied ungleich seltner als heutzutage und hatte deshalb auch mehr zu sagen," "They also used 'du' with each other, that was at the time more unusual for such a difference in age than it is nowadays, and thus says more." Jahn, Otto (1856). W. A. Mozart. Vol. 3. Leipzig: Breitkopf und Härtel. (1858 edition at Google Books (in German)
doi.org
See Brown 1992 for dates and K. numbers. Brown discusses the history of the conjecture that K. 168–173 were influenced by Haydn, and argues against it at length. Brown, Peter (1992). "Haydn and Mozart's 1773 Stay in Vienna: Weeding a Musicological Garden". The Journal of Musicology. 10 (2): 192–230. doi:10.2307/763612. JSTOR763612.
Thomson 1976, p. 28 writes, "[T]he close friendship between the two composers makes it almost certain that this was due to Mozart's influence". Thomson, Katherine (1976). "Mozart and Freemasonry". Music & Letters (57): 25–46. doi:10.1093/ml/LVII.1.25.
See Brown 1992 for dates and K. numbers. Brown discusses the history of the conjecture that K. 168–173 were influenced by Haydn, and argues against it at length. Brown, Peter (1992). "Haydn and Mozart's 1773 Stay in Vienna: Weeding a Musicological Garden". The Journal of Musicology. 10 (2): 192–230. doi:10.2307/763612. JSTOR763612.