# 253

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## Number

$253$ (two hundred and fifty-three) is:

$11 \times 23$

The $15$th integer after $7$, $13$, $19$, $35$, $38$, $41$, $57$, $65$, $70$, $125$, $130$, $190$, $205$, $223$ the decimal representation of whose square can be split into two parts which are each themselves square:
$253^2 = 64 \, 009$; $6400 = 80^2$, $9 = 3^2$

The $22$nd triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $136$, $153$, $171$, $190$, $210$, $231$:
$253 = \ds \sum_{k \mathop = 1}^{22} k = \dfrac {22 \times \paren {22 + 1} } 2$