169

Number
$169$ (one hundred and sixty-nine) is:


 * $13^2$


 * The $13$th square number after $1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144$:
 * $169 = 13 \times 13$


 * The $8$th centered hexagonal number after $1, 7, 19, 37, 61, 91, 127$:
 * $169 = 1 + 6 + 12 + 18 + 24 + 30 + 36 + 42 = 8^3 - 7^3$


 * The $2$nd centered hexagonal number after $1$ which is also square:
 * $169 = 8^3 - 7^3 = 13^2$


 * The $34$th lucky number:
 * $1$, $3$, $7$, $9$, $13$, $15$, $21$, $\ldots$, $115$, $127$, $129$, $133$, $135$, $141$, $151$, $159$, $163$, $169$, $\ldots$


 * The $5$th square lucky number:
 * $1, 9, 25, 49, 169, \ldots$


 * The $11$th positive integer which cannot be expressed as the sum of a square and a prime:
 * $1, 10, 25, 34, 58, 64, 85, 91, 121, 130, 169, \ldots$


 * The smallest square number which is the difference between two cubes:
 * $169 = 8^3 - 7^3$


 * Can be expressed as the sum of $n$ non-zero squares for all $n$ from $1$ to $155$.


 * The $21$st powerful number after $1, 4, 8, 9, 16, 25, \ldots, 108, 121, 125, 128, 144$


 * $1! + 6! + 9! = 363 \, 601$, $3! + 6! + 3! + 6! + 0! + 1! = 1454$, $1! + 4! + 5! + 4! = 169$

Also see

 * 169 as Sum of up to 155 Squares
 * Sum of Factorials of Digits of 169
 * Squares which are Difference between Two Cubes