A number 
with an even number 
of digits formed by multiplying
a pair of 
-digit numbers (where the digits
are taken from the original number in any order) 
and 
together. Pairs of trailing zeros are not allowed. If 
is a vampire number, then 
and 
are called its "fangs." Examples of vampire numbers
include
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(1)
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(2)
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(3)
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(4)
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(5)
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(6)
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(7)
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(OEIS A014575). The 8-digit vampire numbers are 10025010, 10042510, 10052010, 10052064, 10081260, ... (OEIS A048938)
and the 10-digit vampire numbers are 1000174288, 1000191991, 1000198206, 1000250010,
... (OEIS A048939). The numbers of 
-digit vampires are 0, 7, 148, 3228, ... (OEIS A048935).
Vampire numbers having two distinct pairs of fangs include
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(8)
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(9)
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(10)
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(OEIS A048936).
Vampire numbers having three distinct pairs of fangs include
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(11)
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(OEIS A048937).
The first vampire numbers with four pairs of fangs are
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(12)
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(13)
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(14)
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(15)
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and
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(16)
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(17)
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(18)
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(19)
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and the first vampire number with five pairs of fangs is
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(20)
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(21)
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(22)
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(23)
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(24)
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(J. K. Andersen, pers. comm., May 4, 2003).
General formulas can be constructed for special classes of vampires, such as the fangs
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(25)
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(26)
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giving the vampire
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(27)
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(28)
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(29)
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(30)
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where 
denotes 
with the digits reversed (Roush and Rogers 1997-1998).
Pickover (1995) also defines pseudovampire numbers, in which the multiplicands have different numbers of digits.
