# Unknown symbol '#' in set

I am reading a text on Complexity theory. There is a set whose notation I cannot understand:

"Let $\sum$ = {0,1,#}"

From the context, and given that the book is used computer science courses, it seems like $\sum$ is the set which contains all combinations of bit strings up to length '#'.

In the text, a subset of $\sum$ is being used to define a set of palindromes in a Turing machine.

Another notation that I don't understand is in the following expression:

$x \in \{0,1\}^{n/4}$

What is the purpose of the $n/4$?

• @Willie Wong: Thanks for that edit, was unaware of that tag. – ADB Jul 9 '13 at 17:56
• stackexchange is organized as FAQ. You should better split it to two questions. – Jonas Stein Sep 20 '14 at 11:37

$\Sigma = \{0,1,\#\}$ means the set of these 3 symbols, which one usually calls alphabet in complexity theory. And $x\in \{0,1\}^{n/4}$ means that $x$ is a word of length $n/4$ over the alphabet $\{0,1\}$. These are exactly the same definitions as in mathematics, as 'word' means exactly the same as finite sequence which is a function from $\frac{n}{4} \rightarrow \{0,1\}$!