Logical Symbols

　　A ⇒ B means if A is true then B is also true; if A is false then nothing is said about B.

　　→ may mean the same as ⇒ (the symbol may also indicate the domain and codomain of a function; see table of mathematical symbols).

　　⊃ may mean the same as ⇒ (the symbol may also mean superset).

　　A ⇔ B means A is true if B is true and A is false if B is false.

　　The statement ¬A is true if and only if A is false.

　　The statement A ∧ B is true if A and B are both true; else it is false.

　　The statement A ∨ B is true if A or B (or both) are true; if both are false, the statement is false.

　　∀ x: P(x) means P(x) is true for all x.

　　∃ x: P(x) means there is at least one x such that P(x) is true.

　　∃! x: P(x) means there is exactly one x such that P(x) is true.

　　x ⊢ y means y is derived from x.

(编辑：张剑乔)