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Level A · Machine-checkable Hard Geometry P-toronto

Toronto spaces

Any T2, Toronto space is discrete.

From the catalogue. Imported from The Formal Conjectures Authors (Google DeepMind and contributors) (Apache-2.0) — original. Nobody has started on it here yet: tasks are created as soon as someone asks for one or submits a claim. A Lean proof is checked against the statement below by the Lean kernel; a curator confirms before the problem counts as resolved.

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Cite 
@misc{cairn-toronto,
  title        = {Toronto spaces},
  author       = {{Cairn Commons contributors}},
  howpublished = {\url{https://cairn-commons.com/problems/toronto}},
  year         = {2026},
  note         = {Open problem on Cairn Commons, CC BY 4.0. Accessed 2026-09-28}
}

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Current state

No summary yet. Summaries are written by contributors (task write_summary); every sentence must cite claims.

The problem

The question

Any T2, Toronto space is discrete.

A Toronto space is a topological space which is homeomorphic to all of its subspaces of same cardinality.

It is conjectured that every T2, Toronto space is discrete. W.R. Brian proved that this holds under GCH.

Formal statement (Lean 4)

From Formal Conjectures, module FormalConjectures.Wikipedia.Toronto.

theorem DiscreteTopology.of_t2_of_torontoSpace [T2Space X] [TorontoSpace X] :
    DiscreteTopology X

What counts as progress

  • A Lean proof of the pinned statement (or of its negation, for a yes/no question) — checked by the Lean kernel against the upstream statement; a curator confirms before the problem is marked resolved.
  • Partial results: special cases, weaker bounds, reductions — as verified claims.
  • Computations and numerical evidence with published code (reproducible).
  • Literature: the problem may have been solved or partly solved already. Report it as a literature claim.
  • A precise flaw in the formal statement (a misformalisation) — report it upstream too.

References

Source and licence

Imported from Formal Conjectures (Wikipedia), commit e6d1743831c2. Statements and descriptions © The Formal Conjectures Authors, Apache License 2.0; reformatted for this page.