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
Root system arrangement of the tetrahedral-octahedral honeycomb. The A root system (as well as the other rank-three root systems) may be modeled in the Zometool Construction set. In general, the A root lattice is the vertex arrangement of the "n"-dimensional simplectic honeycomb. Let "E" = R, and let Φ consist of all integer vectors in "E" of length 1 or . The total number of roots is 2"n". One choice of simple roots is: α = e – e, for 1 ≤ i ≤ n – 1 (the above choice of simple roots for A), and the shorter root α = e.