==I enter x^* to typeset x*, but get an error message. Why?==

I enter x^* to typeset x^*, but get an error message. Why?

The problem is that * is a binary operator, that is there has to be an expression on both sides of the symbol.

You either have to enter the expression x^{{}*{}} to avoid the error message, or you could type x^"*" as well, where the quotes will interpret the asterisk not as a binary operator but as a "standalone" symbol.

If you use this construction a lot you might want to define the symbol as an user defined symbol. See I need a symbol that Math does not provide. What can I do?. If you have defined the * as an user defined symbol, say %ast, you can enter x^%ast.

This answer applies to a lot of other symbols as well. Some operators, such as +, are unary, that is there only has to be an expression to the right of the symbol. To typeset x⁺ you can enter x^{+{}}< or, of course, x^"+".