22

Number
$22$ (twenty-two) is:


 * $2 \times 11$


 * The $4$th pentagonal number after $1, 5, 12$:
 * $22 = 1 + 4 + 7 + 10 = \dfrac {4 \left({3 \times 4 - 1}\right)} 2$


 * The $3$rd hexagonal pyramidal number after $1, 7$:
 * $22 = 1 + 6 + 15$


 * The $8$th semiprime after $4, 6, 9, 10, 14, 15, 21$:
 * $22 = 2 \times 11$


 * The number of digits in its factorial:
 * $22! = 1 \, 124 \, 000 \, 727 \, 777 \, 607 \, 680 \, 000$
 * which has $22$ digits


 * $22$ is a palindromic number whose square is also palindromic:


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


 * The $10$th even number after $2, 4, 6, 8, 10, 12, 14, 16, 20$ which cannot be expressed as the sum of $2$ composite odd numbers.


 * The smallest positive integer which can be expressed as the sum of $2$ odd primes in $3$ ways:
 * $22 = 19 + 3 = 17 + 5 = 11 + 11$


 * The $11$th positive integer which is not the sum of $1$ or more distinct squares:
 * $2, 3, 6, 7, 8, 11, 12, 15, 18, 19, 22, \ldots$

Also see

 * Square of Small-Digit Palindromic Number is Palindromic