Analysis of information sources in references of the Wikipedia article "Natural number" in English language version.
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: ISBN / Date incompatibility (help) Morash, Ronald P. (1991). Bridge to Abstract Mathematics: Mathematical proof and structures (Second ed.). Mcgraw-Hill College. ISBN 978-0-07-043043-3 – via Google Books....the set of natural numbers is closed under addition... set of natural numbers is closed under multiplication
Addition of natural numbers is associative.
A perfect number is that which is equal to the sum of its own parts.In definition VII.3 a "part" was defined as a number, but here 1 is considered to be a part, so that for example 6 = 1 + 2 + 3 is a perfect number.