Concept information
Preferred term
untouchable number
Definition
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In mathematics, an untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer. That is, these numbers are not in the image of the aliquot sum function. Their study goes back at least to Abu Mansur al-Baghdadi (circa 1000 AD), who observed that both 2 and 5 are untouchable.
(Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Untouchable_number)
Broader concept
In other languages
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French
URI
http://data.loterre.fr/ark:/67375/PSR-GJNBQKR4-C
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