280 (number)

This article covers the number 280. For the year, see 280.
← 279 280 281 →
Cardinaltwo hundred eighty
Ordinal280th
(two hundred eightieth)
Factorization23 × 5 × 7
Greek numeralΣΠ´
Roman numeralCCLXXX, cclxxx
Binary1000110002
Ternary1011013
Senary11446
Octal4308
Duodecimal1B412
Hexadecimal11816

280 (two hundred [and] eighty) is the natural number after 279 and before 281.

In mathematics

The denominator of the eighth harmonic number,[1] 280 is an octagonal number.[2] 280 is the smallest octagonal number that is a half of another octagonal number.

There are 280 plane trees with ten nodes. As a consequence of this, 18 people around a round table can shake hands with each other in non-crossing ways, in 280 different ways (this includes rotations).

References

  1. ^ "Sloane's A002805 : Denominators of harmonic numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-28.
  2. ^ "Sloane's A000567 : Octagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-28.
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