Catherine is really upset that Hal doesn’t believe her. She had waited to show off the proof until she found someone that she could trust. Up until this moment, Hal and Catherine’s budding relationship was steadily building. But by not believing her, Hal destroys their relationship. While Hal may have good reason to suspect that Catherine didn’t write the proof—the handwriting looks like Robert’s and Catherine has only a little formal education—his skepticism also appears to be motivated by sexist thinking. It’s suggested that one of the reasons that Hal doesn’t believe that Catherine wrote the proof is that he doesn’t think that she, a woman, could write something so innovative. He condescends to her, dismissing her abilities and implying that she isn’t really a mathematician like he is. The only person that Hal thinks could write the proof is Robert, a man. If Catherine did indeed write the proof, then Hal’s sexist stereotyping threatens her chance of being recognized for her work. Additionally, if Catherine is the author, then she and her father are even more similar than the audience knew—her ingenious work looks like his.