1. Quantity symbol:

I, 2+i, a, x, natural logarithm base e, pi.

2. Operation symbol:

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

3. Relationship symbols:

For example, = is equal sign, ≈ is approximate sign, ≠ is unequal sign, > is greater than sign,