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Reading a book on algorithms. Can someone explain the meaning of the mathematical symbol ∃?
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Page 1. Math 295. Handout on Shorthand The phrases “for all”, “there exists”, and “such that” are used so frequently in mathematics that we have found it useful to adopt the following shorthand. The symbol ∀ means “for all” or “for any”. The symbol ∃ means “there exists”.
∩ The symbol ∩ means intersection. Given two sets S and T, S ∩ T is used to denote the set {x|x ∈ S and x ∈ T}. For example {1,2,3}∩{3,4,5} = {3}. \ The symbol \ means remove from a set.
⇒ (the implies sign) means “logically implies that”. (E.g., “if it's raining, then it's pouring” is equivalent to saying “it's raining ⇒ it's pouring.”) The history of this symbol is unclear. ⇐⇒ (the iff sign) means “if and only if” and is used to connect logically equivalent statements.
A quantifier is a word that usually goes before a noun to express the quantity of the object; for example, a little milk. Most quantifiers are followed by a noun, though it is also possible to use them without the noun when it is clear what we are referring to. For example, Do you want some milk? – Just a little.
It is called a quantifier. It means "there exists".
When used in an expression such as
∃x s.t. x > 0 It means "There exists a number x such that x is greater than 0."
Its counterpart is ∀, which means "for all". It's used like this:
∀x, x > 0 Which means "For any number x, it is greater than 0."
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It is the "existential quantifier" as opposed to the upside-down A (∀) which means "universal quantifier." It should be read as "there exists" or "for some". It is a predication that means that some relation or property holds true for at least one object in the domain.
Examples:
An integer n is composite if ∃ integer m such that m > 1 and m < n with n divisible by m.
An integer n is prime if ∀ integer m such that m > 1 and m < n it is true that n is not divisible by m.
A function f is continuous on a metric space (X, d) if ∀x∀ε>0∃δ>0 | ∀y d(x, y) < δ => d(f(x), f(y)) < ε
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