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Problem 37

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Truncatable primes

The number $3797$ has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: $3797$, $797$, $97$, and $7$. Similarly we can work from right to left: $3797$, $379$, $37$, and $3$.

Find the sum of the only eleven primes that are both truncatable from left to right and right to left.

NOTE: $2$, $3$, $5$, and $7$ are not considered to be truncatable primes.

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可截素数

$3797$有着奇特的性质。它本身是一个素数；如果从左往右逐一截去数字，剩下的仍然都是素数：$3797$、$797$、$97$和$7$；如果从右往左逐一截去数字，剩下的也仍然都是素数：$3797$、$379$、$37$和$3$。

如果一个素数满足，无论从左往右还是从右往左逐一截去数字，剩下的仍然都是素数，则称之为可截素数。已知总共有十一个可截素数，求这些数的和。

注意：$2$、$3$、$5$和$7$不被视为可截素数。

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