631

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Number

$631$ (six hundred and thirty-one) is:

The $115$th prime number

The smaller of the $10$th pair of primes whose prime gap is $10$:

$641 - 631 = 10$

The $15$th centered hexagonal number after $1$, $7$, $19$, $37$, $61$, $91$, $127$, $169$, $217$, $271$, $331$, $397$, $469$, $547$:

$631 = \ds 1 + \sum_{k \mathop = 1}^{15 - 1} 6 k = 15^3 - 14^3$

Also see

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