Off subject but on Wikipedia Grothendieck says Riemann was his only main or most admired mathematician. Do their fields match up for this to make sense?
Grothendieck did a lot of work in areas that were built off of Riemann's work.
So, for example, Riemann came up with Riemann surfaces, which were the inspiration for Riemann manifolds. Finding ways to generalize that was a major starting point for algebraic geometry. Which Grothendieck did a lot of work in.
As an example, Grothendieck's first famous theorem in algebraic geometry was the https://en.wikipedia.org/wiki/Grothendieck%E2%80%93Riemann%E.... Which finds an appropriate generalization of a result originally due to Riemann in a much more limited context.
Side note. When the name Riemann appears on HN, it almost always is in reference to the Riemann hypothesis. Which is a conjecture in number theory. This might mislead you into thinking that Riemann was a number theorist. He was not. His total contribution to number theory was a single paper where he used his complex analysis techniques to prove the prime number theorem, and made his famous conjecture.
The only other context that most on HN will know the name from was the Riemann integral. Which is the main integral that most who take Calculus will know.
