Tom Marlow, from Saffron Walden in Essex, has been looking at the obscure subject of doubly truncatable primes. For example, taking the prime 1373 (and allowing 1 to be classed as a prime number) he finds that successively curtailing it first from the left and then from the right, produces the series 373 73 3, 137 13 and 1.
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These six extra numbers are all prime, so Tom has designated 1373 as a seven-fold prime. Strangely enough, if these four digits are rearranged to give firstly 3137, and secondly 7331, then in both cases double truncation provides a series of primes. So these two new numbers are also seven-fold in character. These three four-digit primes seem to be the only ones of their size that have this property.
All this leads to Mr Marlow asking: "Which is the only five-digit prime that is double truncatable, ie to provide a nine-fold property?"
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