Sergei Sobolev is a person who lived a long time ago and was very smart. He was born in Russia and he studied a lot about math and physics. He was so good at these things that he became a professor and wrote lots of important papers about them.
One thing Sergei was really interested in was figuring out ways to help people solve big problems using math. He spent a lot of time thinking and working on these kinds of problems and came up with some really cool ideas that other smart people still use today.
Even though he lived a long time ago, Sergei's work has helped us learn a lot about the world and solve problems that seemed impossible. He is remembered as one of the smartest and most important people in the history of math and physics.