 During his twenty @-@ year absence from the public between 1853 and 1873 Alkan produced many of his most notable compositions , although there is a ten @-@ year gap between publication of the Op. 35 studies and that of his next group of piano works in 1856 and 1857 . Of these , undoubtedly the most significant was the enormous Opus 39 collection of twelve studies in all the minor keys , which contains the Symphony for Solo Piano ( numbers four , five , six and seven ) , and the Concerto for Solo Piano ( numbers eight , nine and ten ) . The Concerto takes nearly an hour in performance . Number twelve of Op. 39 is a set of variations , Le <unk> d <unk> ( Aesop 's Feast ) . The other components of Op. 39 are of a similar stature . Smith describes Op. 39 as a whole as " a towering achievement , gathering ... the most complete manifestation of Alkan 's many @-@ sided genius : its dark passion , its vital rhythmic drive , its pungent harmony , its occasionally <unk> humour , and , above all , its uncompromising piano writing . " 
