Wednesday, September 10, 2008

20

20 = 22 x 5.

20 is the smallest number that cannot be prefixed or followed by one digit to form a prime.

20 has a representation as a sum of two squares: 20 = 42 + 22.

20 is the fourth tetrahedral number.

20 is 22 in base 9.

An icosahedron is a polyhedron with 20 triangular faces. The prefix icos comes from the Greek word for “twenty.”

2 comments:

pjcostello said...

20 is a Niven number. The sum of the digits divides the number.

20 is a semiperfect number. There is some set of its divisors which sum to 20 (1+4+5+10=20).

Brian said...

20 is (1^2)(2^2)(3^2)-(4^2)
20 is 2+3+4+5+6
20 is 6^2-4^2
20 is (6-4)(6+4)
20 is (1+1)(.1111^-1+1)
20 is 2^2+2^4
20 is 3^3-3^2+3^1-3^0
20 is 4+4^2
20 is 5^2-5
20 is 6^2-(6x2)-(6-2)
20 is -7^0-7^1+7^2-7x3
20 is 8(8^(1/3))+8/(8^(1/3))
20 is 99/9 + 9
20 is log10(10^20)