"Lightning Calculators" are something of an allied art to magic. (Arthur Benjamin has performed at the Magic Castle, for example).
Those interested in the subject may find THIS to be of interest.
Addresses new and interesting links to other sites (not listed on the Genii website) that merit attention.
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I did a "lightning calculation" act in my 'teens.' Some of the stuff I did way back then is in MATHEMATICAL WIZARDRY.