The sign of (1) quantity: I, 2+I, A, X, natural logarithm base E, pi ∏.

(2) Operation symbols: such as plus sign (+), minus sign (-), multiplication sign (× or), division sign (÷ or/), union of two sets (∩), intersection (∩), root sign () and logarithm (log, lg, ln).

(3) relational symbols: for example, = is an equal sign, ≈ or ≈ is an approximate symbol, ≠ is an unequal sign, > is a greater than sign, and it is a greater than sign.