In 2013, the world over, there were dozens of initiatives to commemorate the bicentennial of the death of Lagrange. For the most part, such commemorations were academic conferences, but attempts were also made to make Lagrange more accessible to the public in general. The effort required to make a mathematician known beyond the restricted circle of experts is daunting enough to merit special notice and praise.

In Paris, at the École Polytechnique from 23 September to 15 December there was an exhibition, curated by Frédéric Brechenmacher, which displayed the mementos of Lagrange conserved in the archives of the École. In Turin, the Academy of Sciences organised, from 19 September to 23 November, another exhibition, which was noteworthy in its demonstration of the flavour, we might say, of Lagrange’s epoch. The editors of the Lettera Matematica also wished to contribute to this laudable work of popularisation, and invited various experts in the mathematics of the 1,700 s to share some of the aspects of their research.

If you ask a mathematician what nationality Lagrange was, in ninety percent of the cases they would reply unhesitatingly that the great mathematician was French. In reality, Lagrange belonged to both Italy and to France (and a little to Germany as well). It was to take this fact into account that here are gathered authors from different countries; the international perspective that results seems to us to be particularly stimulating.

At the beginning, we gave ourselves the objective of covering all branches of mathematics to which Lagrange had contributed. We must confess that we have not been successful in that, but the fault is not ours. Rather, it is the fault of Lagrange, who dealt with all of the mathematics known in his day. If we had followed the criterion of completeness, this special issue would have grown into a volume of several hundred pages. Rather than go into every small detail, each author has limited his or her discussion to a glimpse of the riches that are hidden in the Oeuvres of Lagrange. The portrait of Lagrange that emerges from this mosaic is, in certain respects, new and surprising. We are used to considering Lagrange as little more than a label attached to some theorems that have been passed down through generations of students. In the past, some have even theorised that such a view is essentially correct, postulating that what we are dealing with is a typical case of the mathematician closed within himself. However, modern historians, digging deeper into documents, bringing to light previously unpublished correspondence and re-examining the sequence of theorems, have reconstructed a multi-dimensional figure. Lagrange had his likes and dislikes; he crossed Europe; he was a direct witness to the most important political events of his day; he conversed with ministers, philosophers and scientists. Like Franklin, Rousseau and Mozart, he is a valuable guide for orienting ourselves among the historic structures of the late 1,700 s.

Happy reading.