7

This number is a prime.

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The number of points and lines on the minimal finite projective (or Fano) plane. [Poo Sung]

De Morgan suspected Shanks's calculation of π up to 707 digits was wrong because of the lack of 7s contrary to other digits. [Poo Sung]

(77 + 777)/(7 + 7) = 61 is prime. Note that seven 7's are used and 6 + 1 = 7. [Poo Sung]

There is a persistent rumor that the philosopher G.W.F. Hegel (1770-1831) provided a logical proof within his doctoral dissertation that there could only be 7 planets in the solar system. [Poo Sung]

The first exotic differentiable structure was a 7-dimensional hypersphere. It was found by Milnor (1956) using the calculus of octonions. [Poo Sung]

7 is the smallest number that cannot be represented as a sum of 3 squares. [Poo Sung]

The game Tetris uses 7 one-sided tetrominoes (referred to as I, J, L, O, S, T, and Z). [Poo Sung]

7 = 98532/14076 is the unique pandigital fraction expression including 0. [Poo Sung]