Ramanujan to Hardy: from the first to the last letter. (Ramanujan an Hardy: Vom ersten bis zum letzten Brief.)

Recently invited speaker at the cycle `Un texte, un mathématicien', held at the Bibliothèque nationale de France, Don Zagier changed the title to `Three texts, one mathematician', namely Godfrey Hardy. The latter had intended to write three books: I) An introduction to Calculus II) The famous `A mathematician's apology', III) A volume on his collaboration with Ramanujan: Exchange by mail with the Indian mathematician still staying in Madras and then direct common research activities during Ramanujan's stay in Cambridge.\par Reading the unfinished presentation of III) constitutes a fascinating task