What does ||x|| mean in math? (What does two vertical bars on either side of a mathematical expression mean?)

∥x∥ is the norm of x (i.e. the length of the vector x)

What is the mathematical symbol that looks like a capital V with a line through it:

∀ Denotes universal quantification and is read "for all". If E is a logical predicate, ∀xE means that E is true for all possible values of the variable x

What is the mathematical cymbol that looks like an arrow made from an equals sign and a 'greater than' sign:

⇒  Denotes material conditional, and is read as "implies". If P and Q are logical predicates,P⇒ Q means that if P is true, then Q is also true. It is often used improperly in plain text as an abbreviation of "implies".

What is the mathematical symbol that looks like a back to fron capital E?:

∈ Denotes set membership, and is read "in" or "belongs to". That is, x ∈ S means that x is an element of the set S

1. Borrow stock from someone for an agreed amount of time and a set fee.
2. Sell borrowed stock to someone else (preferably at high price)
3. Buy the same type of stock from someone (preferably at low price) to repay initial lender within the initially agreed amount of time.

     . ^ $ * + ? { } [ ] \ | (

• This happens if you use a set command in an input channel but there is only one value.

	input:
		set val(f) from f_ch

The solution is to remove the set command:

	input:
		val(f) from f_ch

illegal string body character after dollar sign;
   solution: either escape a literal dollar sign "\$5" or bracket the value expression "${5}" @ line 3170, column 43.

This error occurs when you forget to escape a $ sign in a script block.