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 p. 18). This proof constructively exhibits a discrepancy between and any rational. "Proof": For the given equation, there are only six possible combinations of oddness and evenness for whole-number values of formula_18 and formula_19 that produce a whole-number value for formula_24. A simple enumeration of all six possibilities shows why four of these six are impossible. Of the two remaining possibilities, one can be proven to not contain any solutions using modular arithmetic, leaving the sole remaining possibility as the only one to contain solutions, if any. The fifth possibility (both formula_18 and formula_19 odd and formula_24 even) can be