Today is 28 day of the year.
28 = 2 ^2 x 7
28 = 2 + 3 + 5 + 7 + 11, i.e the sum of the first five primes.
28 = 1 + 2 + 4 + 7 + 14, i.e. the aliquot sum equals 12 which makes 12 a perfect number.
28 is a Keith number because a Fibonacci like sequence starting with 2 and 8 includes the number 28:
2, 8, 10, 18, 28, 46 ...
28 is a triangular number.
28 is a member of two primitive, Pythagorean triples (28, 45, 53) and (28, 195, 197)
28 is a happy number.
28 is a member of the Padovan sequence as was 21 exactly a week ago.
28 is a member of the Y-toothpick sequence. There are some nice illustrations here and here.
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