[5] When he had completed the first two of the three movements, he wrote to his friend Pierre Bernac: On 18 June 1957, the public premiere was given at the Strasbourg Music Festival by the flautist Jean-Pierre Rampal, for whom it had been written, with the composer at the piano.
[8] The musicologist Malcolm MacDonald writes of the sonata: The movement starts in 24 (♩= 82) with an opening four-bar phrase with a descending theme, beginning with a broken triad of demisemiquavers around high E and declining to the G above middle C. The piano's right-hand part interweaves arpeggiated semiquavers over a pedal in the left hand.
[11] MacDonald comments that the opening "makes clear the composer’s elegiac intentions", and other analysts write of the "poignancy" of the principal theme, despite the seemingly vivacious tempo.
Wilfrid Mellers comments that the reappearance of the first theme in an unexpected key makes it clear that Poulenc is not following sonata form but is using "a subtle ternary structure".
[12] After a slightly faster middle section there is a recapitulation of a kind with, in Mellers's words, "enharmonic ambiguities that justify the 'malinconico' of the directive", and: The flute part is technically demanding in this movement, with frequent trills and demisemiquaver tonguing.
In its song-like tune Poulenc acknowledged there were echoes of Sister Constance in Dialogues des Carmélites but the effect here is purely lyrical – an "infinite melody" (mélodie infinie) – with none of the drama of the opera.
[17] The fast music culminates in a long trill on high G, followed by a brief silence from which there comes, unexpectedly, a quotation from the dotted rhythm "mélancolique" theme of the first movement's middle section, in its original key of F sharp minor.
[18] The music gathers speed again and the sonata sprints to a double fortissimo finish, "strictement en mesure sans ralentir" ("strictly in time without any slowing down").
Among recordings singled out in a BBC Radio 3 survey in 2015 are those by William Bennett with Clifford Benson; Sharon Bezaly with Ronald Brautigam; Patrick Gallois with Pascal Rogé; Emmanuel Pahud with Le Sage; Ileana Ruhemann with Kathron Sturrock; and Adam Walker with James Baillieu.
[23] In 1976 the flautist James Galway asked the English composer Sir Lennox Berkeley, a good friend of Poulenc's for many years, to orchestrate the sonata.
[It] adds a worthwhile new dimension to the music: the long-drawn sad melodic lines played by the flute, especially in the first two movements, are complemented in a way not possible on the piano".