feat(ErdosProblems): 260

FormalConjectures/ErdosProblems/260.lean

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+/-
+Copyright 2025 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
+
+/-!
+# Erdős Problem 260
+
+*Reference:* [erdosproblems.com/260](https://www.erdosproblems.com/260)
+-/
+
+namespace Erdos260
+
+open Filter SummationFilter
+
+def an_div_n_tendsto_infinity (an : ℕ → ℝ) : Prop :=
+    Tendsto (fun n => an n / (n : ℝ)) atTop atTop
+
+noncomputable def sum_of_sequence (an : ℕ → ℝ) : ℝ :=
+∑'[conditional ℕ] n, (an n / 2 ^ (an n))
+
+@[category research open, AMS 11]
+theorem erdos_260 (a : ℕ → ℝ) (h : StrictMono a) (h2 : an_div_n_tendsto_infinity a) :
+    Irrational (sum_of_sequence a) := by sorry
+
+end Erdos260