16

Number
$16$ (sixteen) is:


 * $2^4$


 * The $4$th square number after $1, 4, 9$:
 * $16 = 4 \times 4$


 * The $5$th powerful number after $1, 4, 8, 9$


 * The $2$nd fourth power after $1$:
 * $16 = 2 \times 2 \times 2 \times 2$


 * The $9$th Ulam number after $1, 2, 3, 4, 6, 8, 11, 13$:
 * $16 = 3 + 13$


 * The $1$st square number to be the sum of two triangular numbers in two different ways:
 * $16 = 10 + 6 = 15 + 1$


 * The $5$th almost perfect number after $1, 2, 4, 8$:
 * $\sigma \left({16}\right) = 31 = 2 \times 16 - 1$


 * The $8$th even number after $2, 4, 6, 8, 10, 12, 14$ which cannot be expressed as the sum of $2$ composite odd numbers.


 * The $11$th after $1, 2, 4, 5, 6, 8, 9, 12, 13, 15$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.

Also see

 * Definition:Hexadecimal Numbers