Task: Question Answering
Input (English): Passage: Alkan's large scale Duo (in effect a sonata) Op. 21 for violin and piano (dedicated to Chrétien Urhan) and his Piano Trio Op. 30 appeared in 1841. Apart from these, Alkan published only a few minor works between 1840 and 1844, after which a series of virtuoso works was issued, many of which he had played at his successful recitals at Érard and elsewhere; these included the Marche funèbre (Op. 26), the Marche triomphale  (Op. 27) and Le chemin de fer (also published, separately, as Op. 27). In 1847 appeared the Op. 31 Préludes and his first large-scale unified piano work, the Grande sonate Les quatre âges (Op. 33). The sonata is structurally innovative in two ways; each movement is slower than its predecessor, and the work anticipates the practice of progressive tonality, beginning in D major and ending in G♯ minor. Dedicated to Alkan Morhange, the sonata depicts in its successive movements its 'hero' at the ages of 20 (optimistic), 30 ("Quasi-Faust", impassioned and fatalistic), 40 (domesticated) and 50 (suffering: the movement is prefaced by a quotation from Aeschylus's Prometheus Unbound). In 1848 followed Alkan's set of 12 études dans tous les tons majeurs Op. 35, whose substantial pieces range in mood from the hectic Allegro barbaro (no. 5) and the intense Chant d'amour-Chant de mort (Song of Love – Song of Death) (no. 10) to the descriptive and picturesque L'incendie au village voisin (The Fire in the Next Village) (no. 7).A number of Alkan's compositions from this period were never performed and have been lost. Among the missing works are some string sextets and a full-scale orchestral symphony in B minor, which was described in an article in 1846 by the critic Léon Kreutzer, to whom Alkan had shown the score. Kreutzer noted that the introductory adagio of the symphony was headed "by Hebrew characters in red ink ... This is no less than the verse from Genesis: And God said, Let there be light: and there was light." Kreutzer opined that, set beside Alkan's conception, Joseph Haydn's Creation was a "mere candle (lampion)." A further missing work is a one-act opera, mentioned frequently in the French musical press of 1846-7 as being shortly to be produced at the Opéra-Comique, which however never materialized. Alkan also referred to this work in a letter of 1847 to the musicologist François-Joseph Fétis, stating that it had been written "a few years ago." Its subject, title and librettist remain unknown. 
Question: What is the full name of the musicologist that the musician who created Piano Trio Op. 30 wrote a letter to referring to a missing one-act opera?
Output (English): François-Joseph Fétis.