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Concept# 45 (number)

Summary

45 (forty-five) is the natural number following 44 and preceding 46.
In mathematics
Forty-five is the smallest odd number that has more divisors than n+1, and that has a larger sum of divisors than n+1. It is the sixth positive integer with a square-prime prime factorization of the form p^{2}q, with p and q prime, and first of the form 3^{2} \times q.
45 has an aliquot sum of 33 that is part of an aliquot sequence composed of five composite numbers (45, 33, 15, 9, 4, 3, 1, and 0), all of-which are rooted in the 3-aliquot tree. This is the longest aliquot sequence for an odd number up to 45.
Forty-five is the sum of all single-digit decimal digits: 0+1+2+3+4+5+6+7+8+9=45. It is, equivalently, the ninth triangle number.
Forty-five is also the fourth hexagonal number and the second hexadecagonal number, or 16-gonal number. It is also the second smallest triangle number (after 1 and 10

Official source

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