The final movement, marked Allegro risoluto, begins with contrapuntal meanderings before breaking out into a full fugue similar to its composer's Grosse Fuge.
It was the inspiration for the later Piano Sonata No. 2 of Boulez, who elaborated on its contrapuntal form.  The first movement of the piece, Allegro, introduces fortissimo B-flat chords, and the very short Scherzo parodies the first.  The third movement, one of the longest of any solo piece up to the time, is an Adagio sostenuto that is said to have inspired much Romantic-era music.  The final movement, marked Allegro risoluto, begins with contrapuntal meanderings before breaking out into a full fugue similar to its composer's Grosse Fuge. FTP, name this piano sonata by Beethoven whose name describes both the instrument and the piece's difficulty.
It was the inspiration for the later Piano Sonata No. 2 of Boulez, who elaborated on its contrapuntal form.  
Hammerklavier or Beethoven's Piano Sonata No. 29