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.
"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.