280 (number) explained

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).

Integers from 281 to 289

281

See main article: 281 (number).

282

See main article: 282 (number).

283

See main article: 283 (number).

284

See main article: 284 (number).

285

See main article: 285 (number).

286

See main article: 286 (number).

287

See main article: 287 (number).

288

See main article: 288 (number).

289

See main article: 289 (number).

Notes and References

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