I _think_ he calls them pernicious because of the ambiguity. 2,3,4,12 is also a "subsequence of natural numbers", but probably not what was meant by 2,3,...,12

I _think_ he calls them pernicious because of the ambiguity. 2,3,4,12 is also a "subsequence of natural numbers", but probably not what was meant by 2,3,...,12. You might counter that ... means the complete subsequence, but what do you mean by "complete"? If I write 2,4,6,8,...,24, you probably want the ... to mean 10,12,14,...[1], not 9,10,11,12,... Basically by the time you make ... precise, you are better off just writing the mathematical notation. [1] See what I did there? ;)