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 The number can also be expressed by taking the Taylor series of a trigonometric function. For example, the series for gives The Taylor series of with and using the double factorial gives The convergence of this series can be accelerated with an Euler transform, producing It is not known whether can be represented with a BBP-type formula. BBP-type formulas are known for and , however. The number can be represented by an infinite series of Egyptian fractions, with denominators defined by 2th terms of a Fibonacci-like recurrence relation a(n)=34a(n-1)-a(n-2), a(0)=0, a(1)=6. The square root of two has the following continued