216 (number)

← 215 216 217 →
Cardinaltwo hundred sixteen
Ordinal216th
(two hundred sixteenth)
Factorization23 × 33
Divisors1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 27, 36, 54, 72, 108, 216
Greek numeralΣΙϚ´
Roman numeralCCXVI, ccxvi
Binary110110002
Ternary220003
Senary10006
Octal3308
Duodecimal16012
HexadecimalD816

216 (two hundred [and] sixteen) is the natural number following 215 and preceding 217. It is a cube, and is often called Plato's number, although it is not certain that this is the number intended by Plato.

In mathematics

Visual proof that 33 + 43 + 53 = 63

216 is the cube of 6, and the sum of three cubes: It is the smallest cube that can be represented as a sum of three positive cubes,[1] making it the first nontrivial example for Euler's sum of powers conjecture. It is, moreover, the smallest number that can be represented as a sum of any number of distinct positive cubes in more than one way.[2] It is a highly powerful number: the product of the exponents in its prime factorization is larger than the product of exponents of any smaller number.[3]

Because there is no way to express it as the sum of the proper divisors of any other integer, it is an untouchable number.[4] Although it is not a semiprime, the three closest numbers on either side of it are, making it the middle number between twin semiprime-triples, the smallest number with this property.[5] Sun Zhiwei has conjectured that each natural number not equal to 216 can be written as either a triangular number or as a triangular number plus a prime number; however, this is not possible for 216. If the conjecture is true, 216 would be the only number for which this is not possible.[6]

There are 216 ordered pairs of four-element permutations whose products generate all the other permutations on four elements.[7] There are also 216 fixed hexominoes, the polyominoes made from 6 squares, joined edge-to-edge. Here "fixed" means that rotations or mirror reflections of hexominoes are considered to be distinct shapes.[8]

In other fields

216 is one common interpretation of Plato's number, a number described in vague terms by Plato in the Republic. Other interpretations include 3600 and 12960000.[9]

There are 216 colors in the web-safe color palette, a color cube.[10]

See also

References

  1. ^ Sloane, N. J. A. (ed.). "Sequence A066890 (Cubes that are the sum of three distinct positive cubes)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  2. ^ Sloane, N. J. A. (ed.). "Sequence A003998 (Numbers that are a sum of distinct positive cubes in more than one way)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  3. ^ Sloane, N. J. A. (ed.). "Sequence A005934 (Highly powerful numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  4. ^ Sloane, N. J. A. (ed.). "Sequence A005114 (Untouchable numbers, also called nonaliquot numbers: impossible values for the sum of aliquot parts function)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  5. ^ Sloane, N. J. A. (ed.). "Sequence A202319 (Lesser of two semiprimes sandwiched each between semiprimes thus forming a twin semiprime-triple)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  6. ^ Sun, Zhi-Wei (2009). "On sums of primes and triangular numbers". Journal of Combinatorics and Number Theory. 1 (1): 65–76. arXiv:0803.3737. MR 2681507.
  7. ^ Sloane, N. J. A. (ed.). "Sequence A071605 (Number of ordered pairs (a,b) of elements of the symmetric group S_n such that the pair a,b generates S_n)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  8. ^ Sloane, N. J. A. (ed.). "Sequence A001168 (Number of fixed polyominoes with n cells)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  9. ^ Adam, J. (February 1902). "The arithmetical solution of Plato's number". The Classical Review. 16 (1): 17–23. doi:10.1017/S0009840X0020526X. JSTOR 694295. S2CID 161664478.
  10. ^ Thomas, B. (1998). "Palette's plunder". IEEE Internet Computing. 2 (2): 87–89. doi:10.1109/4236.670691.