Etalagexpo - Mike Kramer
24-02-06 / 23-03-06
**2² · (2³ - 1)**
It is a composite number, its proper divisors being 1, 2, 4, 7 and 14, making it a perfect number and a harmonic divisor number
28 is the second perfect number. as a perfect number, it is related to the mersenne prime 7, since 2² · (2³ - 1) = 28. the next perfect number is 496, the previous being 6
28 is a triangular number, a hexagonal number and a centered nonagonal number. 28 is the sum of the first five prime numbers
It appears in the padovan sequence, preceded by the terms 12, 16, 21 (it is the sum of the first two of these).
It is also a keith number, because it recurs in a fibonacci-like sequence started from its base 10 digits: 2, 8, 10, 18, 28...
28 is the ninth and last number in early indian magic square of order 3
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