I'm looking for a science fiction short story. I think it is a couple of pages in length. A father mathematician bemoans his mentally challenged child who appears to have Down's Syndrome, doesn't talk and just stands in front of his father's desk impassively staring as his father works in his notebook on the elusive solution to an extremely difficult problem in advanced mathematics. The brilliant father poignantly reflects on the cruel irony of his son's predicament and his sadness and disappointment is profound. As the father gazes out the window of his study, distracted by his thoughts and with tears in his eyes, his son quietly picks up a pencil and, in child-like fashion, scribbles in the notebook, just as he has seen his father do. But when the father looks more closely, he realizes his son’s writing is a solution to the problem the father could not solve, only written upside down, from the viewing perspective of his son. The story appeared in a collection of science fiction short stories and is probably at least two decades, if not three, old.
Thanks to user14111, who suggested I post on BookSleuth Forum, where I received the following answer from Lee W. Kuivinen: It's "Problem Child" by Arthur Porges. It's in 10TH ANNUAL EDITION: THE YEAR'S BEST S-F edited by Judith Merril (1965).
He entered the study, and walked to the desk. The top sheet lay there, mocking him—but what was this? The last equation was crossed out, and above it there was a long line of pencil marks. Almost like mathematical symbols, but not—by God, upside down!
Bewildered, he reversed the sheet. For a moment the writing still seemed without content, then Kadar felt his heart contract like a clenched fist. A new integral transform—powerful, elegant, and startlingly original. It would crack the tough kernel of the problem as lightning shatters an oak.
He looked up, wild-eyed. Paul met his gaze squarely. The slender throat was working; the lips moved.
"Like that . . . it has to be like that. Other way . . . the pattern is ugly," the boy mumbled, his voice a queer, high-pitched stammer, as if he had to claw the words out of a diaphragm never before used.