215 (number)

215 (two hundred [and] fifteen) is the natural number following 214 and preceding 216.

In mathematics

• 215 is a composite number and a semiprime (${\displaystyle =5\times 43}$).
• ${\displaystyle 215=(3!)^{3}-1}$.
• 215 is the second smallest integer (after 5) such that ${\displaystyle n^{2}-17}$ is twice a square: ${\displaystyle 215^{2}-17=2\times 152^{2}}$.^[1]
• 215 is a vertically symmetric number on a calculator display (sequence A053701 in the OEIS).
• There are 215 sequences of four integers, counting re-arrangements as distinct, such that the sum of their reciprocals is 1.^[2] These are:
  • 24 arrangements of (2,3,7,42), (2,3,8,24), (2,3,9,18), (2,3,10,15), (2,4,5,20) and (2,4,6,12).
  • 12 arrangements of (3,3,4,12), (3,4,4,6), (2,3,12,12), (2,4,8,8) and (2,5,5,10).
  • 6 arrangements of (3,3,6,6).
  • 4 arrangements of (2,6,6,6).
  • 1 arrangement of (4,4,4,4).

In other fields

• 215 Oenone is a main belt asteroid.
• E215 is the E number of Sodium ethyl para-hydroxybenzoate.
• There are several highways numbered 215.

215 is also:

• The Dewey Decimal Classification for Science and religion.
• The year AD 215 or 215 BC
• 215 is often used as slang for marijuana, from California Proposition 215, legalizing it for medical use.
• The first area code of metropolitan Philadelphia, Pennsylvania

References

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This page was last edited on 22 April 2024, at 17:32