feat(OEIS): A056777

FormalConjectures/OEIS/56777.lean

@@ -0,0 +1,82 @@
+/-
+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
+
+open scoped ArithmeticFunction
+
+/-!
+# Conjectures associated with A056777
+
+A056777 lists composite numbers $n$ satisfying both $\varphi(n+12) = \varphi(n) + 12$ and
+$\sigma(n+12) = \sigma(n) + 12$.
+
+The conjectures state identities connecting A056777 and prime quadruples (A007530), as
+well as congruences satisfied by the members of A056777.
+
+*References:* [oeis.org/A056777](https://oeis.org/A056777)
+-/
+
+namespace OeisA056777
+
+/-- A composite number $n$ is in the sequence A056777 if it satisfies both
+$\varphi(n+12) = \varphi(n) + 12$ and $\sigma(n+12) = \sigma(n) + 12$. -/
+def a (n : ℕ) : Prop :=
+  ¬n.Prime ∧ 1 < n ∧
+  Nat.totient (n + 12) = Nat.totient n + 12 ∧
+  σ 1 (n + 12) = σ 1 n + 12
+
+/-- A number $n$ comes from a prime quadruple $(p, p+2, p+6, p+8)$ if
+$n = p(p+8)$ for some prime $p$ where $p$, $p+2$, $p+6$, $p+8$ are all prime. -/
+def comesFromPrimeQuadruple (n : ℕ) : Prop :=
+  ∃ p : ℕ, p.Prime ∧ (p + 2).Prime ∧ (p + 6).Prime ∧ (p + 8).Prime ∧
+    n = p * (p + 8)
+
+/-- $65$ is in the sequence A056777. -/
+@[category test, AMS 11]
+theorem a_65 : a 65 := by
+  refine ⟨?_, by norm_num, ?_, ?_⟩
+  · native_decide
+  · native_decide
+  · native_decide
+
+/-- $209$ is in the sequence A056777. -/
+@[category test, AMS 11]
+theorem a_209 : a 209 := by
+  refine ⟨?_, by norm_num, ?_⟩ <;> native_decide
+
+/-- Numbers coming from prime quadruples are in the sequence A056777. -/
+@[category undergraduate, AMS 11]
+theorem a_of_comesFromPrimeQuadruple {n : ℕ} (h : comesFromPrimeQuadruple n) : a n := by
+  sorry
+
+/-- All members of the sequence A056777 come from prime quadruples. -/
+@[category research open, AMS 11]
+theorem comesFromPrimeQuadruple_of_a {n : ℕ} (h : a n) : comesFromPrimeQuadruple n := by
+  sorry
+
+/-- Numbers coming from prime quadruples satisfy $n \equiv 65 \pmod{72}$. -/
+@[category undergraduate, AMS 11]
+theorem mod_72_of_comesFromPrimeQuadruple {n : ℕ} (h : comesFromPrimeQuadruple n) :
+    n % 72 = 65 := by
+  sorry
+
+/-- Numbers coming from prime quadruples satisfy $n \equiv 9 \pmod{100}$. -/
+@[category undergraduate, AMS 11]
+theorem mod_100_of_comesFromPrimeQuadruple {n : ℕ} (h : comesFromPrimeQuadruple n) :
+    n % 100 = 9 := by
+  sorry
+
+end OeisA056777