Cube (arithmetic)
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In arithmetic and algebra, the cube of a number n is its third power — the result of multiplying it by itself two times:
- n3 = n × n × n.
This is also the volume formula for a geometric cube of side length n, giving rise to the name.
The term cube or cube number is often used to refer to a perfect cube i.e. a number that is the cube of a positive integer.
The series of perfect cubes starts as follows:
- 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, ...
Unlike perfect squares, perfect cubes do not have a small number of possibilities for the last two digits. Except for cubes divisible by 5, where only 25 and 75 can be the last two digits, any pair of digits with the last digit odd can be a perfect cube. With even cubes, there is considerable restriction, for only 00, o2, e4, o6 and e8 can be the last two digits of a perfect cube. (o stands for any odd digit and e for any even digit).
It is, however, easy to show that most numbers are not perfect cubes because all perfect cubes must have digital root 1, 8 or 9. Moreover, the digital root of any number's cube can be determined by the remainder it gives when divided by 3:
- If the number is divisible by 3, its cube has digital root 9;
- If it has a remainder of 1 when divided by 3, its cube has digital root 1;
- If it has a remainder of 2 when divided by 3, its cube has digital root 8.
The inverse operation of finding a number whose cube is n is called extracting the cube root of n. It determines the side of the cube of a given volume. It is also n raised to the one-third power.
Every positive integer can be written as the sum of nine cubes or fewer; see Waring's problem. This upper limit of nine cubes cannot be reduced because, for example, 23 cannot be written as the sum of fewer than nine cubes :-
- 23 = 23 + 23 + 13 + 13 + 13 + 13 + 13 + 13 + 13
Each cube number n3 is also the sum of the first n centered hexagonal numbers, although representing a different shape.
External links
- http://www.alpertron.com.ar/FCUBES.HTM is a Java applet that decomposes an integer number not congruent to 4 or 5 (mod 9) into a sum of four cubes.ja:立方数