> I would like to have confirmations from other primary sources then just said hero and people who worship him.

Sure, I'll bite. This is a passage about Stallman's performance in Harvard's Math 55, which has a reputation as the hardest single undergraduate math course in the US:

To earn the right to boast, however, Stallman, Chess, and the other SHP alumni had to get through Math 55. Promising four years worth of math in two semesters, the course favored only the truly devout. "It was an amazing class," says David Harbater, a former "math mafia" member and now a professor of mathematics at the University of Pennsylvania. "It's probably safe to say there has never been a class for beginning college students that was that intense and that advanced. The phrase I say to people just to get it across is that, among other things, by the second semester we were discussing the differential geometry of Banach manifolds. That's usually when their eyes bug out, because most people don't start talking about Banach manifolds until their second year of graduate school."

Starting with 75 students, the class quickly melted down to 20 by the end of the second semester. Of that 20, says Harbater, "only 10 really knew what they were doing." Of that 10, 8 would go on to become future mathematics professors, 1 would go on to teach physics.

"The other one," emphasizes Harbater, "was Richard Stallman."

Seth Breidbart, a fellow Math 55 classmate, remembers Stallman distinguishing himself from his peers even then.

"He was a stickler in some very strange ways," says Breidbart. There is a standard technique in math which everybody does wrong. It's an abuse of notation where you have to define a function for something and what you do is you define a function and then you prove that it's well defined. Except the first time he did and presented it, he defined a relation and proved that it's a function. It's the exact same proof, but he used the correct terminology, which no one else did. That's just the way he was."

It was in Math 55 that Richard Stallman began to cultivate a reputation for brilliance. Breidbart agrees, but Chess, whose competitive streak refused to yield, says the realization that Stallman might be the best mathematician in the class didn't set in until the next year. "It was during a class on Real Analysis, which I took with Richard the next year," says Chess, now a math professor at Hunter College. "I actually remember in a proof about complex valued measures that Richard came up with an idea that was basically a metaphor from the calculus of variations. It was the first time I ever saw somebody solve a problem in a brilliantly original way."

Chess makes no bones about it: watching Stallman's solution unfold on the chalkboard was a devastating blow. As a kid who'd always taken pride in being the smartest mathematician the room, it was like catching a glimpse of his own mortality. Years later, as Chess slowly came to accept the professional rank of a good-but-not-great mathematician, he had Stallman's sophomore-year proof to look back on as a taunting early indicator.

"That's the thing about mathematics," says Chess. "You don't have to be a first-rank mathematician to recognize first-rate mathematical talent. I could tell I was up there, but I could also tell I wasn't at the first rank. If Richard had chosen to be a mathematician, he would have been a first-rank mathematician."

Source: https://www.oreilly.com/openbook/freedom/ch04.html