The man on the right has given his name to a popular topic of discussion in mathematics. Identify.

Cracked by: shrik, shrey, dineshk, Rogi, Dibyo, jowens, Manish Achuth, sandman, Debasish, krudebox, arman, rohanquizzer, Thejas V R, udupendra, Siddarth Pai, Ananth, Sumanth Patlolla, Raghuvansh, raklodramA, abinash sharma, aditthya, Shwetha Maiya, Aravind, Anjul and Atul Mathew