Together with his 9th Symphony, Gustav Mahler’s masterwork Das Lied von der Erde is somewhat enigmatic; not a symphony, not a song cycle, not a cantata, with the unbelievable, incredible, essential “Der Abschied” as probably the highest point in Mahler’s work. Soon, to be performed in Copenhagen with the Danish National Symphony Orchestra. On the program is another Farewell Symphony, Schubert’s immortal and no less enigmatic “Unfinished” symphony.