85 (eighty-five) is the natural number following 84 and preceding 86.

In mathematics

85 is:

  • the product of two prime numbers (5 and 17), and is therefore a semiprime of the form (5.q) where q is prime.
  • specifically, the 24th Semiprime, it being the fourth of the form (5.q).
  • together with 86 and 87, forms the second cluster of three consecutive semiprimes; the first comprising 33, 34, 35.
  • with a prime aliquot sum of 23 in the short aliquot sequence (85,23,1,0).
  • an octahedral number.
  • a centered triangular number.
  • a centered square number.
  • a decagonal number.
  • the smallest number that can be expressed as a sum of two squares, with all squares greater than 1, in two ways, 85 = 9<sup>2</sup> + 2<sup>2</sup> = 7<sup>2</sup> + 6<sup>2</sup>.
  • the length of the hypotenuse of four Pythagorean triangles.
  • a Smith number in decimal.

In other fields

<!--

Note: please do not add things which simply and non-notably have this number of members.

For example the number of pages or chapters in a book, episodes or seasons of a series, subdivisions in a state, rockets of a particular type, etc.

-->

  • The radix of the Ascii85 (sometimes called Base85) binary-to-text encoding
  • Part of the assignation for the Toyota AE85, commonly referred to as an "eight-five".

References

  • 85
  • Jorge Stolfi, The Hollywood Constant.