# What

• Wikipedia Glossary of Mathematical Symbols

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``

• What is short selling?

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.

• %(#ffffff)[What is the complete list of Python regular expression metacharacters?]

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

• What does strata insurance cover in the case of water damage?
Strata insurers will cover the cost to repair water damage to insured property but generally exclude repairs cost related to finding and fixing the leak, as it is generally considered the owners responsibility to maintain their property and such repairs relate to general maintenance or wear and tear

• What is the cause of the error "Unknown method invocation `checkWarn` on TaskProcessor type" in NextFlow

• 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
``````

• What is the cause of the following error when trying to run a NextFlow script:

``````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.