feat(Paper): conjecture about cardinality of Lindelöf spaces

FormalConjectures/Paper/CardinalityLindelof.lean

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+Copyright 2026 The Formal Conjectures Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-/
+import FormalConjectures.Util.ProblemImports
+
+/-!
+# Conjecture about cardinality of Lindelöf spaces
+
+The conjecture asks for a Lindelöf space where all singletons are G_δ sets
+and which has cardinality > 𝔠.
+
+This is Problem 1 in https://www.math.md/files/basm/y2013-n2-3/y2013-n2-3-(pp37-46).pdf.pdf
+
+*Reference:*
+* [Selected Old Open Problems in General Topology](https://www.math.md/files/basm/y2013-n2-3/y2013-n2-3-(pp37-46).pdf.pdf)
+  by A. V. Arhangel’skii
+
+-/
+
+open Cardinal
+
+namespace CardinalityLindelof
+
+/--
+A space where all singletons are Gδ sets.
+-/
+class HasGδSingletons (X : Type*) [TopologicalSpace X] : Prop where
+  isGδ_singleton : ∀ ⦃x : X⦄, IsGδ {x}
+
+/-- Singletons are Gδ in First-Countable T₁ Spaces- -/
+@[category test, AMS 54]
+instance HasGδSingletons.of_t1Space_firstCountableTopology (X : Type*) [TopologicalSpace X]
+    [FirstCountableTopology X] [T1Space X] : HasGδSingletons X where
+  isGδ_singleton := IsGδ.singleton
+
+/--
+Is there a Lindelöf space with singletons as Gδ sets with cardinality greater than the continuum?
+-/
+@[category research open, AMS 54]
+theorem HasGδSingletons.lindelof_card :
+    ∃ (X : Type) (_ : TopologicalSpace X), HasGδSingletons X ∧ LindelofSpace X ∧ 𝔠 < #X := by
+  sorry
+
+-- TODO: under additional axioms (consistent with ZFC), such a space exists.
+
+end CardinalityLindelof