CorollaryStatement: AI-adaptedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passverified 2026-08-06 (claude-opus-5)

How statement and proof provenance work

The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.

Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.

These labels describe origin, not correctness: citations and verification chips remain separate evidence.

There is no set $y$ with $x \in y \leftrightarrow \forall s\,(s \in \varnothing \to x \in s)$ , so $\bigcap \varnothing$ is undefined

Statement

There is no set $y$ such that, for every set $x$ ,

$x \in y \leftrightarrow \forall s\,(s \in \varnothing \to x \in s).$

The defining condition for $\bigcap x$ therefore determines no set when $x = \varnothing$ , which is why $\bigcap \varnothing$ is left undefined.

Facts & Assumptions

Given: nothing beyond the results cited below.

[L1]

There is exactly one set with no elements, written $\varnothing$ (There is exactly one set with no elements, written $\varnothing$ ).

[L2]

There is no set $U$ such that $y \in U$ for every set $y$ (There is no set $U$ with $y \in U$ for every set $y$ ).

[L3]

For $x \neq \varnothing$ , $\bigcap x$ is the set whose elements are exactly the sets belonging to every element of $x$ (The intersection $\bigcap x$ of a nonempty set, the binary intersection $a \cap b := \bigcap\{a,b\}$ , and disjointness).

Proof

technique · contradiction

1.1

Suppose there is a set $y$ such that, for every set $x$ , $x \in y$ holds if and only if $x$ belongs to every element of $\varnothing$ .

assume-contra

1.2

$\varnothing$ has no elements, so for every set $x$ the condition " $x$ belongs to every element of $\varnothing$ " is satisfied vacuously.

2.1

Combining, every set $x$ satisfies $x \in y$ .

step 1.1step 1.2

3.1

A set with every set as an element does not exist, so the supposition fails; the condition defining $\bigcap$ therefore determines no set at $\varnothing$ , and $\bigcap \varnothing$ is undefined.

L2L3step 2.1discharge-contradiction∎

Depends on

Used by

FALSE: ⋂ ∅ = ∅ False statement

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 11 results over 5 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.

Sources

B. Kaya, MATH 320 Set Theory (METU), §1.2 (standard reference, not scraped)
C. Wilson, A Brief Introduction to ZFC (Chicago REU 2016), §2.3 (standard reference, not scraped)
Intersection (set theory) (Wikipedia) (standard reference, not scraped)

There is no set yyy with x∈y↔∀s (s∈∅→x∈s)x \in y \leftrightarrow \forall s\,(s \in \varnothing \to x \in s)x∈y↔∀s(s∈∅→x∈s), so ⋂∅\bigcap \varnothing⋂∅ is undefined