Three is one of my favorite numbers! Hopefully the rest of this page will show you why.

3 is the first odd prime number, as it's only divisible by 1 and 3. 3 is a Mersenne Prime: (2^2)-1 = 3. Mersenne primes must be of the form (2^p)-1 where p is prime. This also means that 3 factors into a perfect number: 6, as all even perfect numbers are of the form M * 2^(p-1) where M is a mersenne prime.

Three is a triangular number, for obvious reasons.

3 roughly approximates to *e*, and less roughly into π.

Three points that aren't collinear can determine a plane or circle.

And of course, there are three spatial dimensions.

- In Binary, 3 is 11.
- In Ternary, 3 is 10. Ternary is considered the most "efficient" base, as it's radix economy is closest to
*e*. - Quaternary, Septimal, decimal or any other base greater than a multiple of 3 has a divisibility trick: if a number's digits add to a multiple of 3, the original number is divisible by 3. This works if you substitute 3 for another number N, and the bases with numbers one greater than a multiple of N. For 3, this means most bases have an easy divisibility trick for it, including decimal and even hex, which also work for 9 and 5 respectively.
- 6 is divisible by 3, so all numbers in seximal ending in 0 or 3 are threeven.
- 9 is 3^2, so two ternary digits correspond to a nonary digit.
- 12 is divisible by 3, so all numbers in dozenal ending in 0, 3, 6, or 9 are threeven.
- 27 is 3^3, so three ternary digits correspond to one digit in trinonary.
- In Negabinary, 3 is 111

and -3 is 1101. - In Negaternary, 3 is 120

and -3 is 10.

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