The book is now available! 731
(another Prime Pages' Curiosity)
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+ 731 is composite, being the product of the primes 17 and 43, but 7331, 73331, 733331 and 7333331 are all prime. This is the only three-digit composite with this property.

+ The largest composite number such that deleting any digit(s) will never yield another composite number, i.e. only yields primes or 1. [Hasler]

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