perfect number

noun Mathematics.
a positive number that is equal to the sum of all positive integers that are submultiples of it, as 6, which is equal to the sum of 1, 2, and 3.


Origin:
1350–1400; Middle English

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Collins
World English Dictionary
perfect number
 
n
an integer, such as 28, that is equal to the sum of all its possible factors, excluding itself

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
perfect number  
A positive integer that equals the sum of all of its divisors other than itself. An example is 28, whose divisors (not counting itself) are 1, 2, 4, 7, and 14, which added together give 28.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

perfect number

a positive integer that is equal to the sum of its proper divisors. The smallest perfect number is 6, which is the sum of 1, 2, and 3. Other perfect numbers are 28, 496, and 8,128. The discovery of such numbers is lost in prehistory. It is known, however, that the Pythagoreans (founded c. 525 BC) studied perfect numbers for their "mystical" properties.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Mathematicians define a perfect number as positive integer that is the sum of it's positive divisors excluding the number itself.
Well, where injuries and illnesses are concerned, zero is the perfect number.
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