Te X Input Method

typing          | result
x_1             | x₁
x^2             | x²
\infty          | ∞
\times          | ×
\div            | ÷
\pm             | ±
\angle          | ∠
	        
\ne             | ≠
\ge             | ≥
\le             | ≤
\nleq           | ≰
\ngeq           | ≱
\sim            | ∼
\simeq          | ≃

\sum            | ∑
\prod           | ∏
\emptyset       | ∅
\cap            | ∩ 
\cup            | ∪
\subset         | ⊂
\subseteq       | ⊆
\supset         | ⊃
\supseteq       | ⊇

\forall         | ∀
\exists         | ∃
\nexists        | ∄
\wedge          | ∧
\vee            | ∨
\neg            | ¬

\equiv          | ≡

\in             | ∈
\notin          | ∉
\ni             | ∋

\lfloor         | ⌊            ⌊x⌋
\rfloor         | ⌋
\lceil          | ⌈            ⌈x⌉
\rceil          | ⌉
\vert           | |            |x|
\langle         | 〈
\rangle         | 〉            〈x〉

\Leftrightarrow | ⇔
\leftrightarrow | ↔
\leftarrow      | ←
\Leftarrow      | ⇐
\rightarrow     | →
\Rightarrow     | ⇒

\natural        | ♮
\complement     | ∁

\copyright      | ©

\oe             | œ
\OE             | Œ
\AE             | Æ
\ae             | æ

And some codes to use with M-x insert-ucs-character