50

Number
$50$ (fifty) is:
 * $2 \times 5^2$


 * The $1$st positive integer which can be expressed as the sum of two square numbers in two or more different ways:
 * $50 = 7^2 + 1^2 = 5^2 + 5^2$


 * The $2$nd term of the $1$st $5$-tuple of consecutive integers have the property that they are not values of the divisor sum function $\map {\sigma_1} n$ for any $n$:
 * $\tuple {49, 50, 51, 52, 53}$


 * The $4$th hexagonal pyramidal number after $1$, $7$, $22$:
 * $50 = 1 + 6 + 15 + 28$


 * The $4$th noncototient after $10$, $26$, $34$:
 * $\nexists m \in \Z_{>0}: m - \map \phi m = 50$
 * where $\map \phi m$ denotes the Euler $\phi$ function


 * The $5$th nontotient after $14$, $26$, $34$, $38$:
 * $\nexists m \in \Z_{>0}: \map \phi m = 50$
 * where $\map \phi m$ denotes the Euler $\phi$ function


 * The $31$st positive integer after $2$, $3$, $4$, $7$, $8$, $\ldots$, $33$, $37$, $38$, $42$, $43$, $44$, $45$, $46$, $49$ which cannot be expressed as the sum of distinct pentagonal numbers.

Also see