That is Alexander Grothendieck. He's involved with some abstract algebra I'm way to sober to grok rn. My only knowledge of him is that his stuff basically established category theory as a thing, although apparently that was completely unintentional on his end.

He seems to have been an... interesting character in his later years

Local villagers helped sustain him with a more varied diet after he tried to live on a staple of dandelion soup.