The young, Berlin-born pianist Martin Helmchen, who made his BSO debut in 2011 at Tanglewood with Schumann's Piano Concerto and his subscription series debut in 2015 with Beethoven's Piano Concerto No. 5, Emperor, is now featured in Beethoven's Piano Concerto No. 3, which pays homage to Mozart and Haydn while also exhibiting Beethoven's own intense individuality. Written nearly a century later, the first of Mahler's nine symphonies employs folk-music references and a conventional four-movement form that have their foundations in Haydn's time. Its expanded scope and instrumentation are evidence of the genre's 19th-century transformation as well as Mahler's own expansion of the form.