400

Number
$400$ (four hundred) is:


 * $2^4 \times 5^2$


 * The $20$th square number after $1, 4, 9, 16, 25, 36, \ldots, 225, 256, 289, 324, 361$:
 * $400 = 20^2$


 * The $24$th number, and $3$rd square number after $1, 81$, whose $\sigma$ value is square:


 * The $6$th even integer after $2, 4, 94, 96, 98$ that cannot be expressed as the sum of $2$ prime numbers which are each one of a pair of twin primes.