Describe the set of all odd numbers between $100$ and $200$ using set builder notation I've come across a question in Discrete Mathematics, asking me to use set builder notation to describe the set of all odd numbers between 100 and 200.
The answer I had was:
$$\{ p | p = 2n + 1, n \text{ (all numbers) } [50, 99], 100 < p < 200 \}$$
Although this should technically give the correct answer, the answers in the textbook have:
$$\{x\,|\,100<x<200\text{ and }2\not | x\}$$
I get the first part, however I have no clue what the end means (2 |/ x); what is that symbol called, and does that represent all odd numbers? 

The symbol $\mid$ means 'divides'. Drawing a line through it to get $\not \mid$ means 'does not divide'. As for your answer, it is absolutely correct. There are many equally correct ways to write the set of odd numbers with setbuilder notation.
