what does an a with two dots over it mean
Ä Ä (lower case ä) is a character that represents either a letter from several extended Latin alphabets, or the letter A with an umlaut mark or diaeresis.
Bracket Forms include round (also called "parentheses"), square, curly (also called "braces"), and angle brackets (also called "chevrons"); and various other pairs of symbols.
Limit of a function (the Dirichlet function) has no limit at any x-coordinate.
Alonso Álvarez de Pineda