7/Historical Note

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Historical Note on $7$

The most obvious contemporary social significance of the number $7$ is the number of days in the week, which may have originated from the fact that it corresponds approximately with the number of days between specific phases of the moon.

Rational Diagonal

The number $7$ (seven) was referred to by the ancient Greeks as the rational diagonal of the $5 \times 5$ square.

This was on account of the fact that:

$5^2 + 5^2 = 50 \approx 7^2 = 49$