It varies by operating system.
Most modern operating systems allow you access to a wide range of Unicode characters, and if you can type them you can usually post them. ( +±×∗÷∕⁄<>≤≥=≠≡≅≈∼∝∩∪⊂⊄⊃⊅⊆⊇∈∉∋∍¬⇒⇔∀∃∄∧∴∠⊥∞ℵ√∛∜∂∇ ∫∅⊕⊗a∙b, c⋄d,e⋅f∏∑ %‰‱§¶ℑ℘ℜℕℍℒℓℙℝℤℂℎℏ30°½⅓⅔¼¾ΑΒΓΔΕΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩαβγδεζηθικλμνξοπρσςτυφχψωϑϕϖ
This greatly limits your use of superscripts and subscripts (₀₁₊₂₃₄₋₅₆₍₇₈₉X⁽⁰¹²³⁺⁴⁵⁶⁻⁷⁸⁹⁾⁼ⁿ) so exponentiation is usually written with a caret: x^y, e^(iωt)
Some characters are designed for multi-line presentations, but their use is not recommended except at the far left side.