How to express this statement using quantifiers

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In summary, there is a real number between any two real numbers, assuming that the first number is less than the second number. This can be expressed as "for all y and z, there is some x such that y < x < z" or "there is an x such that for all y and z, y < x < z". However, the first expression is more accurate as the second expression would imply the same x works for all possible values of y and z.
  • #1
rasen58
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There is a real number between any other two real numbers.
I have two ways of writing it
For all y and z there is some x such that y < x < z
OR
There is such an x such that for all y and z, y < x < z

I'm confused as to which one is correct.
 
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  • #2
None of them is fully correct, you must add the assumption that y < z, otherwise there would be a number x such that e.g. 2 < x < 1.

If you add this assumption, the first one is correct, because x depends on y and z. If the second was correct, there would be an x which lies between any numbers y and z such that y < z, so there would be an x such that 0 < x < 1, 1 < x < 2, 55 < x < 971 etc. i.e. the same x would work for all y and z such that y < z.
 
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Thank you! That makes sense!
 

Related to How to express this statement using quantifiers

1. How do I express a statement using quantifiers?

To express a statement using quantifiers, you need to first identify the variables and the domain of discourse. Then, use quantifiers such as "for all" (∀) or "there exists" (∃) to specify the extent of the statement. For example, the statement "All dogs bark" can be expressed using the universal quantifier as ∀x(Dog(x) → Bark(x)), where Dog(x) represents the variable x being a dog and Bark(x) represents the variable x barking.

2. What are the two types of quantifiers?

The two types of quantifiers are universal quantifiers (∀) and existential quantifiers (∃). Universal quantifiers are used to express statements that apply to all elements in a given domain, while existential quantifiers are used to express statements that apply to at least one element in a given domain.

3. How do I use quantifiers in mathematical statements?

In mathematical statements, quantifiers are used to specify the extent of a statement and to clarify what elements the statement applies to. They are commonly used in the form of "for all" (∀) and "there exists" (∃) symbols, followed by the variable and the condition that the variable must satisfy. For example, the statement "There exists an integer x such that x + 2 = 5" can be written as ∃x(x ∈ Z ∧ x + 2 = 5), where Z represents the set of integers.

4. Are quantifiers always necessary in statements?

No, quantifiers are not always necessary in statements. In some cases, the use of quantifiers may be implied or understood without explicitly stating them. However, in formal logic and mathematics, quantifiers are important for clarity and precision in expressing statements.

5. Can I use multiple quantifiers in a single statement?

Yes, you can use multiple quantifiers in a single statement. This is often necessary when expressing complex statements or statements that involve multiple variables. However, it is important to use quantifiers correctly and to specify the domain of discourse for each quantifier to avoid ambiguity.

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