This is an abridged version of Alain Plagne’s article *Yahya ould Hamidoune, The Mauritanian mathematician*, to appear in Combinatorics, Probability and Computing. The full version in pdf format is available here.

« Yahya ould Hamidoune passed away in Paris on Friday March 11, 2011 after a brief illness, leaving insufficient time for his friends and colleagues to express their indebtedness to him for his kindness and generosity, both in mathematics and in everyday life. Yahya was a discreet individual, always looking for the essential rather than the superficial, and certainly did not receive the recognition he deserved. May this modest testimony render justice to this singular man.

Yahya ould Hamidoune was born in October 1947 in Atar, Mauritania. His family belong to the highly literate tribe of Owlad Daymân and his father was a famous encyclopedist, writing among other things the book on Mauritania, an encyclopedia in 42 volumes, *La vie mauritanienne* (Mauritanian Life).

At fifteen, Yahya went to Cairo to study mathematics up to the graduate level. In 1970, he returned to Nouakchott and began teaching at the Lycée National, the most famous high school in Mauritania; at that time, there was no university in Nouakchott. But Yahya needed challenges, and loved playing games. He became the national champion of Mauritanian draughts. He also attained a high level in chess (as I recall, his ELO was about 2200), tarot, bridge and backgammon. At that time, Yahya was also involved in several revolutionary and anti-neocolonialist movements which shook Mauritanian society, and indeed the entire world, in the 70’s. He would pay the price for his involvement, spending several months in jail.

In 1975, Yahya sought a fresh intellectual challenge. He went to France and started his doctoral studies in graph theory with M. Las Vergnas at the University Pierre et Marie Curie (Paris 6). Yahya’s first publication, Sur les atomes d’un graphe orienté, appeared in the Comptes Rendus de l’Académie des sciences in 1977. He soon became an expert on graph connectivity and obtained his PhD (Quelques problèmes de connexité dans les graphes orientés) in February 1978. He was admitted to the CNRS (Centre National de la Recherche Scientifique) as a full-time researcher in 1979, and joined C. Berge’s group at Paris 6.

Until the middle of the 80’s, Yahya worked almost exclusively on connectivity problems. […] After reading H. B. Mann’s book *Addition Theorems*, […] Yahya realized that certain results of his on graph connectivity generalize, in a disguised form, some classical combinatorial results of additive number theory (at that time, this body of work was not yet called additive combinatorics). The isoperimetric method was born. Yahya proceeded to investigate, in a systematic way, the classical theorems of the theory, those due to Cauchy-Davenport, Chowla, Olson, Mann, Shepherdson, Shatrowsky, Vosper, Kneser, Kemperman, …. He found new proofs, improvements, generalizations, and many applications of these theorems.

Yahya’s most famous result is certainly his 1991 proof with J. A. Dias da Silva of a conjecture due to P. Erdös and H. A. Heilbronn on restricted addition of sets modulo a prime p. The paper appears in the Bulletin of the London Mathematical Society (1994) under the slightly cryptic title *Cyclic spaces for Grassmann derivatives and additive theory*. […]

Very recently, Yahya answered in a brilliant way a question of T. Tao dealing with a non-commutative version of Kneser’s theorem. Indeed, as I can certify, Yahya was able to answer the question very rapidly after reading it. It is likely that this or similar results already existed in his mind before the question was asked. He simply needed an opportunity to write it down! […]

But Yahya was not only a mathematician, especially when in Mauritania, where he travelled two or three times a year. There, he was a personality, known to — and loved by — a large part of the population. When his remains arrived in Nouakchott in the middle of the night, about five thousand people were waiting for him at the airport. In fact, Yahya was well-known in Mauritania as a militant for democracy and ecology. He was a deeply honest citizen who did not fear to fight against corruption, a fight which occasionally provoked death threats. One of his preferred combats was protecting the Parc National du Banc d’Arguin — a huge Mauritanian bird sanctuary protected by UNESCO — notably in 2005 against a petroleum company. In fact, Yahya was the main character in a documentary, *Between the oil and the deep blue sea*, which shed light on this fight. […] »

Alain Plagne

Centre des Mathématiques Laurent Schwartz, Ecole Polytechnique, 91128 Palaiseau, France.

**Other pages of tribute to Yahya ould Hamidoune**

- Page by M. Las Vergnas (in French)
- Page by Alain Plagne (in French)