We had a ‘retarded’ man in our little home town who couldn’t take care of himself but who could accurately solve - in his head and without paper to work on- any mathematical problem posed to him. He was a savant. He was well known about town for that, for loudly blurting out the change owed to shoppers before the clerk could, and for notifying everyone in a most insistent way of changes in the Daylight Savings Time as if his and everyone elses’ lives depended on it.
Those people USED to have the name: "idiot-savant."
Shades of the movie DELIVERANCE. I remember that the movie featured a banjo-playing "idiot-savant."
I imagine that the "retarded" man is probably in an institution now.