22
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Number
$22$ (twentytwo) is:
 $2 \times 11$
 The $1$st positive integer which can be expressed as the sum of $2$ odd primes in $3$ ways:
 $22 = 19 + 3 = 17 + 5 = 11 + 11$
 The $2$nd integer after $1$ which equals the number of digits in its factorial:
 $22! = 1 \, 124 \, 000 \, 727 \, 777 \, 607 \, 680 \, 000$
 which has $22$ digits
 The $2$nd Smith number after $4$:
 $2 + 2 = 2 + 1 + 1 = 4$
 The $3$rd pentagonal number after $1$, $5$ which is also palindromic:
 $22 = 1 + 4 + 7 + 10 = \dfrac {4 \left({3 \times 4  1}\right)} 2$
 The $3$rd hexagonal pyramidal number after $1$, $7$:
 $22 = 1 + 6 + 15$
 The $3$rd number after $1$, $3$ whose $\sigma$ value is square:

 $\sigma \left({22}\right) = 36 = 6^2$
 The $4$th pentagonal number after $1$, $5$, $12$:
 $22 = 1 + 4 + 7 + 10 = \dfrac {4 \left({3 \times 4  1}\right)} 2$
 The $7$th generalized pentagonal number after $1$, $2$, $5$, $7$, $12$, $15$:
 $22 = \dfrac {4 \left({3 \times 4  1}\right)} 2$
 The $8$th semiprime after $4$, $6$, $9$, $10$, $14$, $15$, $21$:
 $22 = 2 \times 11$
 The $8$th of $35$ integers less than $91$ to which $91$ itself is a Fermat pseudoprime:
 $3$, $4$, $9$, $10$, $12$, $16$, $17$, $22$, $\ldots$
 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 $11$th positive integer which is not the sum of $1$ or more distinct squares:
 $2$, $3$, $6$, $7$, $8$, $11$, $12$, $15$, $18$, $19$, $22$, $\ldots$
 $22$ is a palindromic number whose square is also palindromic:
 $22^2 = 484$
Also see
 Previous ... Next: Numbers Equal to Number of Digits in Factorial
 Previous ... Next: Numbers whose Sigma is Square
 Previous ... Next: Smith Number
 Previous ... Next: Sequence of Palindromic Pentagonal Numbers
 Previous ... Next: Hexagonal Pyramidal Number
 Previous ... Next: Smallest Positive Integer which is Sum of 2 Odd Primes in n Ways
 Previous ... Next: Pentagonal Number
 Previous ... Next: Generalized Pentagonal Number
 Previous ... Next: 91 is Pseudoprime to 35 Bases less than 91
 Previous ... Next: Numbers not Sum of Distinct Squares
 Previous ... Next: Positive Even Integers not Expressible as Sum of 2 Composite Odd Numbers
 Previous ... Next: Semiprime Number
Sources
 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $22$
 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $22$