Beyond “dessins d’enfant”, the theory nowadays referred to as Grothendieck-Teichmüller theory (Galois-Teichmüller in Grothendieck’s manuscripts) may well represent the main new theme in the Esquisse d'un Programme, as confirmed in the Promenade à travers une œuvre (which is part of Récoltes et semailles). Simplifying a great deal one may say that Grothendieck’s main ideas were taken up especially by Y. Ihara, V. Drinfeld and P. Deligne in the mid and late eighties.They derive in large part from the elementary remark that the fundamental group remains the only invariant in classical algebraic topology which is not a priori abelian .Making this remark fruitful probably required the genius of Alexandre Grothendieck . The fact is that out of it Grothendieck-Teichmüller theory (on which we will concentrate) and Anabelian Geometry (including the so-called “section conjecture”) were born.In Grothendieck’s Esquisse, he is dealing with the full étale fundamental group, which is profinite almost...
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