When I was a kid, the term for it was idiot savant, although it's known to be a form of autism now. I never knew just how he was related to me, but he was related to me in some way. Nels lived across the road from me. I don't know whether the story was true or not, but the story was that he had been institutionalized at some point in his life, in a place that was then known as a nut house, although I am pretty sure that's not the politically correct term these days. After escaping several times, he was finally allowed to live where he was living while I was a kid, across the road from me. Nels was a harmless person. Although in his thirties or forties, maybe older, he enjoyed playing kid's games, and was very much a kid in a lot of ways, and not a particularly bright kid -- that is, unless you asked him anything having to do with numbers. He could tell us when all of our cats had been born, how many litters each had had, including the dates and how many kittens were in each litter, and how each cat was related to the other, which is something I couldn't even do with my own family, let alone our cats. More remarkably, you could give him any quantify of numbers, however long, and by the time you got to the last number, he could tell you what they added up to. The same was true of division or multiplication. The moment he was given the last number, he had solved the problem, however many digits were involved. Mathematics was second nature to him, or perhaps it could be called first nature, and he knew dates. Pretty much anything having to do with numbers, but he never understood the concept of hide and seek. He would hide pretty much in the open, and would tell you where he was if you asked him.