280 (number)

This article covers the number 280. For the year, see 280.

← 279 280 281 →
← 200 210 220 230 240 250 260 270 280 290 → List of numbers Integers ← 0 100 200 300 400 500 600 700 800 900 →
Cardinal	two hundred eighty
Ordinal	280th (two hundred eightieth)
Factorization	23 × 5 × 7
Greek numeral	ΣΠ´
Roman numeral	CCLXXX
Binary	1000110002
Ternary	1011013
Senary	11446
Octal	4308
Duodecimal	1B412
Hexadecimal	11816

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

In mathematics

The denominator of the eighth harmonic number, 280 is an octagonal number. 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

Main article: 281 (number)

282

282 = 2·3·47, sphenic number, number of planar partitions of 9

283

283 prime, twin prime with 281, strictly non-palindromic number, 4²⁸³ - 3²⁸³ is prime

284

284 and 220 form the first pair of amicable numbers, as the divisors of 284 add up to 220 and vice versa.

285

285 = 3·5·19, sphenic number, square pyramidal number, Harshad number, repdigit in base 7 (555), vertically symmetric number (sequence A053701 in the OEIS).

286

286 = 2·11·13, sphenic number, tetrahedral number, nontotient.

286 is the smallest even pseudoprime (not divisible by 3) to base 3, which implies 3²⁸⁵ ≡ 1 (mod 286).

287

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), pentagonal number.

288

Main article: 288 (number)

289

289 = 17², centered octagonal number, Friedman number since (8 + 9)² = 289.