Why No Acknowledgement of Pushkin's Play?
The first play to treat this theme was "Моцарт и Сальери" (Mozart and Salieri), by Alexander Pushkin, but I don't think Peter Shaffer ever acknowledged this source. (Of course, there had been rumors that it was Salieri who murdered Mozart out of jealousy, but this Russian play was written less than 40 years after Mozart's death, and was the first serious piece of literature that treated these rumors.)
If anyone has read, or seen, the Pushkin play--in any language--I'd like to know if he/she thinks Pushkin's work was at least partly plagiarized.
Im Arme der Götter wuchs ich groß--Friedrich Hölderlin