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👁️‍🗨️formal logic i review

There exists an x such that for all y...

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

This phrase describes a logical statement involving quantifiers where there is at least one specific element 'x' in a given domain for which a certain condition holds true for every element 'y' in that same or another domain. It highlights the relationship between individual instances and universal generalizations, making it crucial in formal logic for expressing properties that hold true universally under specific conditions.

5 Must Know Facts For Your Next Test

  1. The phrase uses two different quantifiers: the existential quantifier (∃) indicating existence and the universal quantifier (∀) indicating generality.
  2. In this context, 'there exists an x' implies that you can find at least one specific instance that meets the criteria for all instances of 'y'.
  3. The order of quantifiers is significant; switching them changes the meaning of the statement entirely.
  4. This logical structure is often used in proofs to establish the existence of a solution or example based on a universal property.
  5. When interpreting this phrase, it's essential to carefully define the domains for both 'x' and 'y' to avoid ambiguity.

Review Questions

  • How does changing the order of quantifiers affect the meaning of the statement 'there exists an x such that for all y...'?
    • Changing the order of quantifiers drastically alters the meaning of the statement. For example, 'there exists an x such that for all y...' suggests that one specific 'x' can satisfy a condition for every possible 'y'. In contrast, if we reverse it to 'for all y, there exists an x...', it implies that for each individual 'y', there could be different 'x' values satisfying the condition. This distinction is crucial in logic as it impacts how we interpret and prove statements.
  • Explain how 'there exists an x such that for all y...' can be applied in mathematical proofs.
    • 'There exists an x such that for all y...' can be pivotal in mathematical proofs, especially in showing the existence of solutions to equations or inequalities. For instance, if you are proving that there is a maximum value (x) of a function such that it holds true for all values within its domain (y), this structure helps confirm not just existence but also universality under specified conditions. It allows mathematicians to establish properties about functions or sets systematically.
  • Evaluate the implications of using both existential and universal quantifiers together in logical statements.
    • 'There exists an x such that for all y...' presents a complex relationship between individual cases and universal truths. The use of both types of quantifiers enables us to convey nuanced assertions about relationships and properties within sets. For instance, this could mean finding one specific element (x) that upholds a rule applicable universally across another set (y). This framework can lead to powerful conclusions in mathematics and logic, influencing theories related to existence and uniqueness within formal systems.