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 (this includes rotations).

280 is a base 10 Harshad number.

In geography

*List of highways numbered 280

See also the year 280.

281 to 289

Two hundred [and] eighty-one 281 prime, twin prime with 283, Sophie Germain prime, sum of the first fourteen primes, sum of seven consecutive primes (29 + 31 + 37 + 41 + 43 + 47 + 53), Chen prime, Eisenstein prime with no imaginary part, centered decagonal number----Two hundred [and] eighty-two 282 = 2·3·47, sphenic number----Two hundred [and] eighty-three 283 prime, twin prime with 281, strictly non-palindromic number----Two hundred [and] eighty-four 284 amicable number with 220; nontotient. 284 degrees Celsius is the flash point of paper.----Two hundred [and] eighty-five 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 (555).----Two hundred [and] eighty-six 286 = 2·11·13, sphenic number, tetrahedral number, nontotient, also shorthand for the Intel 80286 microprocessor chip----Two hundred [and] eighty-seven 287 = 7·41, sum of three consecutive primes (89 + 97 + 101), sum of five consecutive primes (47 + 53 + 59 + 61 + 67), sum of nine consecutive primes (17 + 19 + 23 + 29 + 31 + 37 + 41 + 43 + 47), Kynea number, pentagonal number, also shorthand for the Intel math coprocessor to the 80286----Two hundred [and] eighty-eight 288 = 2⁵·3², pentagonal pyramidal number, 5 superfactorial, untouchable number, Harshad number, self number, and is two-dosen sets of a dosen, thus being two gross, often told as a pun in maths classes. 288=32=1¹+2²+3³+4⁴----Two hundred [and] eighty-nine 289 = 17², centered octagonal number, Friedman number since (8 + 9)^2 = 289