While this theory is considered exceedingly unlikely by some, it is not so considered by Janina Fialkowska, the pianist who premiered Liszt's Piano Concerto No. 3 in 1990.
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She says she was told by Roch Serra (who was told by the Liszt scholar Professor Milstein, who was told by Vera Timanova, who was told by Sophie Menter herself), that Liszt was indeed the composer of the piece, but he did not want Tchaikovsky to be aware of this.