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 a 2 by 2 matrix If "B" is zero but "A" and "D" are not both zero, one can use This formula will provide two solutions if "A" = "D" or "A" = 0 or "D" = 0, and four otherwise. A similar formula can be used when "C" is zero but "A" and "D" are not both zero. Square root of a 2 by 2 matrix A square root of a 2×2 matrix "M" is another 2 by 2 matrix "R" such that "M" = "R", where "R" stands for the matrix product of "R" with itself. In general there can be no, two,