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Problem 35

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Circular primes

The number, $197$, is called a circular prime because all rotations of the digits: $197$, $971$, and $719$, are themselves prime.

There are thirteen such primes below $100$: $2$, $3$, $5$, $7$, $11$, $13$, $17$, $31$, $37$, $71$, $73$, $79$, and $97$.

How many circular primes are there below one million?

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圆周素数

$197$被称为圆周素数，因为将它逐位轮转所得到的数：$197$、$971$和$719$都是素数。

小于$100$的圆周素数有十三个：$2$、$3$、$5$、$7$、$11$、$13$、$17$、$31$、$37$、$71$、$73$、$79$和$97$。

小于一百万的圆周素数有多少个？

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