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Correct me if I am wrong, but I believe the author made an error. The author wrote:

"A prime from which you can remove numbers and still have a prime is a deletable prime, such as 1987".

However if you remove the 7, then the number is divisible by 2, so the statement: "A prime from which you can remove numbers and still have a prime is a deletable prime" does not apply to 1987.

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> However if you remove the 7, then the number is divisible by 2, so the statement: "A prime from which you can remove numbers and still have a prime is a deletable prime" does not apply to 1987.

A deletable prime doesn't require that the deletion order start from the last digit; the deletion order for 1987 is:

1987 -> 197 -> either 19 or 17

The definition given in the article is an informal shortcut, the complete definition is http://mathworld.wolfram.com/DeletablePrime.html and 1987 is indeed a deletable prime (see http://oeis.org/A080608/b080608.txt for http://oeis.org/A080608).

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