what does an upward arrow mean in math
Knuth's up-arrow notation A single arrow means exponentiation (iterated multiplication), more than one arrow means iterating the operation associated with one less arrow.
Limit of a function (the Dirichlet function) has no limit at any x-coordinate.
Bracket Forms include round (also called "parentheses"), square, curly (also called "braces"), and angle brackets (also called "chevrons"); and various other pairs of symbols.
Exponentiation Any nonzero number raised to the 0 power is 1:[11]