Answer: Jebediah Buxton was an English autistic savant with the skill to calculate very large numbers.
Jebediah Buxton was born in 1707 in a small city named Elmton near Derbyshire. He was the son of a farmer, and became a farmer himself. He didn't exhibit very good skills with words, and wasn't able to write. Buxton did, however, demonstrate a talent for numbers and counting. For example, he was able to mentally calculate the value of a farthing to the 139th power, a number with 39 digits.
He was evaluated by the Royal Academy in London for his mathematical skills. He was apparently able to start a complex calculation, then return to it several years later as if uninterrupted.
The phrase autistic savant refers to a person with autism who also has other talents that far exceed the capabilities of others without autism. Savants are more common among people with autism, with estimates as high as 10%, whereas it is extremely rare in people without autism. The most famous autistic savant is Kim Peek, the man who inspired the movie Rain Man.
Jebediah Buxton was described by Oliver Sacks in his book Anthropologist on Mars in the chapter called "Savants."
You can read more case studies in neuroscience here.