Mental calculators are people with a prodigious ability in some area of mental calculation, such as identifying large primes, multiplying large numbers together or factoring large numbers. Some mental calculators are autistic, with a narrow area of great skill and poor mental development in other directions (see savant). Others are also great mathematicians, linguists, writers...
Mental calculators were in great demand in research centers such as CERN before the advent of modern electronic calculators and computers. See, for instance, the modern book "The Great Mental Calculators" (whose introduction was written by Eberstark).
