280 (number)


280 is the natural number after 279 and before 281.

In mathematics

The denominator of the eighth harmonic number, 280 is an 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.
280 is a base 10 Harshad number.

In geography

See also the year 280.

Integers from 281 to 289

281

282

282 = 2·3·47, sphenic number

283

283 prime, twin prime with 281, strictly non-palindromic number

284

285

285 = 3·5·19, sphenic number, square pyramidal number, Harshad number, also in Star Trek, the total number of Rules of Acquisition, repdigit in base 7.

286

286 = 2·11·13, sphenic number, tetrahedral number, nontotient, also shorthand for the Intel 80286 microprocessor chip.
286 is the smallest even pseudoprime to base 3, which implies 3285 ≡ 1.

287

287 = 7·41, sum of three consecutive primes, sum of five consecutive primes, sum of nine consecutive primes, Kynea number, pentagonal number, also shorthand for the Intel math coprocessor to the 80286

288

289

289 = 172, centered octagonal number, Friedman number since 2 = 289.