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84 is a number that proceeds 83 and preceeds 85.[]



In Mathematics[]

84 is a semiperfect number, being thrice a perfect number, and the sum of the sixth pair of twin primes (41 + 43).

It is the third (or second) dodecahedral number, and the sum of the first seven triangular numbers (1, 3, 6, 10, 15, 21, 28, 36), which makes it the sixth tetrahedral number.

The twenty-second unique prime in decimal, with notably different digits than its preceding (and known following) terms in the same sequence, contains a total of 84 digits.

A hepteract is a seven-dimensional hypercube with 84 penteract 5-faces.

84 is the limit superior of the largest finite subgroup of the mapping class group of a genus g surface divided by g.[citation needed]

Under Hurwitz's automorphisms theorem, a smooth connected Riemann surface X of genus g > 1 will contain an automorphism group Aut (X) = G whose order is classically bound to IGI < 84 (g - 1).

There are 84 zero divisors in the 16-dimensional sedenions .

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