Vampire Number
A number 
with an Even number 
of Digits formed by multiplying a pair of 
-Digitnumbers (where the Digits are taken from the original number in any order) 
and 
together. Pairsof trailing zeros are not allowed. If 
is a vampire number, then and are called its ``fangs.'' Examples ofvampire numbers include
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(Sloane's A014575). There are seven 4-digit vampires, 148 6-digit vampires, and 3228 8-digit vampires. General formulas canbe constructed for special classes of vampires, such as the fangs
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giving the vampire
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where
denotes with the Digits reversed (Roushe and Rogers).Pickover (1995) also defines pseudovampire numbers, in which the multiplicands have different numbers of digits.
References
Pickover, C. A. ``Vampire Numbers.'' Ch. 30 in Keys to Infinity. New York: Wiley, pp. 227-231, 1995. Pickover, C. A. ``Vampire Numbers.'' Theta 9, 11-13, Spring 1995. Pickover, C. A. ``Interview with a Number.'' Discover 16, 136, June 1995. Roushe, F. W. and Rogers, D. G. ``Tame Vampires.'' Undated manuscript. Sloane, N. J. A. Sequence A014575in ``The On-Line Version of the Encyclopedia of Integer Sequences.''http://www.research.att.com/~njas/sequences/eisonline.html.
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