400 (number) third perfect number, a number whose divisors add up to the actual number (1+2+4+8+16+31+62+124+248=496). 497 = 7 × 71, sum of five consecutive primes (89 + 97 + 101 + 103 + 107) 498 = 2 × 3 × 83, sphenic number, untouchable number, admirable number, abundant number A prime number, Chen prime 400 (number) 400 (four hundred) is the natural number following 399 and preceding 401. 400 is the square of 20. 400 is the sum of the powers of 7 from 0 to 3, thus making it a repdigit in base 7 (1111). A circle is divided into
Square root of 2 widely used constant. formula_81 The (approximate) aspect ratio of paper sizes under ISO 216 (A4, A0, etc.) is 1:. This ratio of lengths of the shorter over the longer side guarantees that cutting a sheet in half along a line parallel to its shorter side results in the smaller sheets having the same (approximate) ratio as the original sheet. Proof: Let formula_82 shorter length and formula_83 longer length of the sides of a sheet of paper, with Let formula_85 be the analogue ratio of the halved sheet, then Square root of 2 The square root of 2, or the (1/2)th