feat: When the sum of a nonneg function is zero (#7372)

Also fix implicitness of arguments to `Finset.sum_singleton`.

Mathlib/Algebra/BigOperators/Basic.lean

@@ -8,9 +8,10 @@ import Mathlib.Algebra.Group.Pi
 import Mathlib.Algebra.GroupPower.Lemmas
 import Mathlib.Algebra.Hom.Equiv.Basic
 import Mathlib.Algebra.Ring.Opposite
+import Mathlib.Data.Finset.Powerset
+import Mathlib.Data.Finset.Sigma
 import Mathlib.Data.Finset.Sum
 import Mathlib.Data.Fintype.Basic
-import Mathlib.Data.Finset.Sigma
 import Mathlib.Data.Multiset.Powerset
 import Mathlib.Data.Set.Pairwise.Basic
 
@@ -340,7 +341,7 @@ theorem prod_insert_one [DecidableEq α] (h : f a = 1) : ∏ x in insert a s, f
 #align finset.sum_insert_zero Finset.sum_insert_zero
 
 @[to_additive (attr := simp)]
-theorem prod_singleton : ∏ x in singleton a, f x = f a :=
+theorem prod_singleton (f : α → β) (a : α) : ∏ x in singleton a, f x = f a :=
   Eq.trans fold_singleton <| mul_one _
 #align finset.prod_singleton Finset.prod_singleton
 #align finset.sum_singleton Finset.sum_singleton
@@ -790,7 +791,7 @@ theorem prod_eq_single_of_mem {s : Finset α} {f : α → β} (a : α) (h : a 
         · intro _ H
           rwa [mem_singleton.1 H]
         · simpa only [mem_singleton] }
-    _ = f a := prod_singleton
+    _ = f a := prod_singleton _ _
 #align finset.prod_eq_single_of_mem Finset.prod_eq_single_of_mem
 #align finset.sum_eq_single_of_mem Finset.sum_eq_single_of_mem
 
@@ -1265,8 +1266,7 @@ theorem prod_range_zero (f : ℕ → β) : ∏ k in range 0, f k = 1 := by rw [r
 
 @[to_additive sum_range_one]
 theorem prod_range_one (f : ℕ → β) : ∏ k in range 1, f k = f 0 := by
-  rw [range_one]
-  apply @prod_singleton β ℕ 0 f
+  rw [range_one, prod_singleton]
 #align finset.prod_range_one Finset.prod_range_one
 #align finset.sum_range_one Finset.sum_range_one
 
@@ -1460,6 +1460,13 @@ theorem prod_pow (s : Finset α) (n : ℕ) (f : α → β) : ∏ x in s, f x ^ n
 #align finset.prod_pow Finset.prod_pow
 #align finset.sum_nsmul Finset.sum_nsmul
 
+/-- A product over `Finset.powersetLen` which only depends on the size of the sets is constant. -/
+@[to_additive
+"A sum over `Finset.powersetLen` which only depends on the size of the sets is constant."]
+lemma prod_powersetLen (n : ℕ) (s : Finset α) (f : ℕ → β) :
+    ∏ t in powersetLen n s, f t.card = f n ^ s.card.choose n := by
+  rw [prod_eq_pow_card, card_powersetLen]; rintro a ha; rw [(mem_powersetLen.1 ha).2]
+
 @[to_additive]
 theorem prod_flip {n : ℕ} (f : ℕ → β) :
     (∏ r in range (n + 1), f (n - r)) = ∏ k in range (n + 1), f k := by

Mathlib/Algebra/BigOperators/Order.lean

@@ -202,7 +202,7 @@ theorem prod_eq_one_iff_of_le_one' :
 @[to_additive single_le_sum]
 theorem single_le_prod' (hf : ∀ i ∈ s, 1 ≤ f i) {a} (h : a ∈ s) : f a ≤ ∏ x in s, f x :=
   calc
-    f a = ∏ i in {a}, f i := prod_singleton.symm
+    f a = ∏ i in {a}, f i := (prod_singleton _ _).symm
     _ ≤ ∏ i in s, f i :=
       prod_le_prod_of_subset_of_one_le' (singleton_subset_iff.2 h) fun i hi _ ↦ hf i hi
 #align finset.single_le_prod' Finset.single_le_prod'
@@ -494,7 +494,7 @@ theorem prod_lt_prod_of_subset' (h : s ⊆ t) {i : ι} (ht : i ∈ t) (hs : i 
 theorem single_lt_prod' {i j : ι} (hij : j ≠ i) (hi : i ∈ s) (hj : j ∈ s) (hlt : 1 < f j)
     (hle : ∀ k ∈ s, k ≠ i → 1 ≤ f k) : f i < ∏ k in s, f k :=
   calc
-    f i = ∏ k in {i}, f k := prod_singleton.symm
+    f i = ∏ k in {i}, f k := by rw [prod_singleton]
     _ < ∏ k in s, f k :=
       prod_lt_prod_of_subset' (singleton_subset_iff.2 hi) hj (mt mem_singleton.1 hij) hlt
         fun k hks hki ↦ hle k hks (mt mem_singleton.2 hki)
@@ -682,23 +682,60 @@ end CanonicallyOrderedCommSemiring
 end Finset
 
 namespace Fintype
-
-variable [Fintype ι]
+section OrderedCommMonoid
+variable [Fintype ι] [OrderedCommMonoid M] {f : ι → M}
 
 @[to_additive (attr := mono) sum_mono]
-theorem prod_mono' [OrderedCommMonoid M] : Monotone fun f : ι → M ↦ ∏ i, f i := fun _ _ hfg ↦
+theorem prod_mono' : Monotone fun f : ι → M ↦ ∏ i, f i := fun _ _ hfg ↦
   Finset.prod_le_prod' fun x _ ↦ hfg x
 #align fintype.prod_mono' Fintype.prod_mono'
 #align fintype.sum_mono Fintype.sum_mono
 
+@[to_additive sum_nonneg]
+lemma one_le_prod (hf : 1 ≤ f) : 1 ≤ ∏ i, f i := Finset.one_le_prod' λ _ _ ↦ hf _
+
+@[to_additive] lemma prod_le_one (hf : f ≤ 1) : ∏ i, f i ≤ 1 := Finset.prod_le_one' λ _ _ ↦ hf _
+
+end OrderedCommMonoid
+
+section OrderedCancelCommMonoid
+variable [Fintype ι] [OrderedCancelCommMonoid M] {f : ι → M}
+
 @[to_additive sum_strictMono]
-theorem prod_strictMono' [OrderedCancelCommMonoid M] : StrictMono fun f : ι → M ↦ ∏ x, f x :=
+theorem prod_strictMono' : StrictMono fun f : ι → M ↦ ∏ x, f x :=
   fun _ _ hfg ↦
   let ⟨hle, i, hlt⟩ := Pi.lt_def.mp hfg
   Finset.prod_lt_prod' (fun i _ ↦ hle i) ⟨i, Finset.mem_univ i, hlt⟩
 #align fintype.prod_strict_mono' Fintype.prod_strictMono'
 #align fintype.sum_strict_mono Fintype.sum_strictMono
 
+@[to_additive sum_pos]
+lemma one_lt_prod (hf : 1 < f) : 1 < ∏ i, f i :=
+  Finset.one_lt_prod' (λ _ _ ↦ hf.le _) $ by simpa using (Pi.lt_def.1 hf).2
+
+@[to_additive]
+lemma prod_lt_one (hf : f < 1) : ∏ i, f i < 1 :=
+  Finset.prod_lt_one' (λ _ _ ↦ hf.le _) $ by simpa using (Pi.lt_def.1 hf).2
+
+@[to_additive sum_pos_iff_of_nonneg]
+lemma one_lt_prod_iff_of_one_le (hf : 1 ≤ f) : 1 < ∏ i, f i ↔ 1 < f := by
+  obtain rfl | hf := hf.eq_or_lt <;> simp [*, one_lt_prod]
+
+@[to_additive]
+lemma prod_lt_one_iff_of_le_one (hf : f ≤ 1) : ∏ i, f i < 1 ↔ f < 1 := by
+  obtain rfl | hf := hf.eq_or_lt <;> simp [*, prod_lt_one]
+
+@[to_additive]
+lemma prod_eq_one_iff_of_one_le (hf : 1 ≤ f) : ∏ i, f i = 1 ↔ f = 1 := by
+  simpa only [(one_le_prod hf).not_gt_iff_eq, hf.not_gt_iff_eq]
+    using (one_lt_prod_iff_of_one_le hf).not
+
+@[to_additive]
+lemma prod_eq_one_iff_of_le_one (hf : f ≤ 1) : ∏ i, f i = 1 ↔ f = 1 := by
+  simpa only [(prod_le_one hf).not_gt_iff_eq, hf.not_gt_iff_eq, eq_comm]
+    using (prod_lt_one_iff_of_le_one hf).not
+
+end OrderedCancelCommMonoid
 end Fintype
 
 namespace WithTop

Mathlib/Analysis/Convex/Combination.lean

@@ -330,7 +330,7 @@ theorem convexHull_eq (s : Set E) : convexHull R s =
     (_ : ∑ i in t, w i = 1) (_ : ∀ i ∈ t, z i ∈ s), t.centerMass w z = x } := by
   refine' Subset.antisymm (convexHull_min _ _) _
   · intro x hx
-    use PUnit, {PUnit.unit}, fun _ => 1, fun _ => x, fun _ _ => zero_le_one, Finset.sum_singleton,
+    use PUnit, {PUnit.unit}, fun _ => 1, fun _ => x, fun _ _ => zero_le_one, sum_singleton _ _,
       fun _ _ => hx
     simp only [Finset.centerMass, Finset.sum_singleton, inv_one, one_smul]
   · rintro x ⟨ι, sx, wx, zx, hwx₀, hwx₁, hzx, rfl⟩ y ⟨ι', sy, wy, zy, hwy₀, hwy₁, hzy, rfl⟩ a b ha

Mathlib/Data/Nat/Digits.lean

@@ -552,7 +552,7 @@ theorem sub_one_mul_sum_div_pow_eq_sub_sum_digits
         have ih := ih (w₂' h') w₁'
         simp only [self_div_pow_eq_ofDigits_drop _ _ h, digits_ofDigits p h tl w₁' w₂',
           succ_eq_one_add] at ih
-        have := @sum_singleton _ _ tl.length (fun x => ofDigits p <| tl.drop x) _
+        have := sum_singleton (fun x ↦ ofDigits p <| tl.drop x) tl.length
         rw [← Ico_succ_singleton, List.drop_length, ofDigits] at this
         have h₁ : 1 ≤ tl.length :=  List.length_pos.mpr h'
         rw [← sum_range_add_sum_Ico _ <| h₁, ← add_zero (∑ x in Ico _ _, ofDigits p (tl.drop x)),

Mathlib/NumberTheory/Divisors.lean

@@ -373,10 +373,8 @@ theorem sum_properDivisors_dvd (h : (∑ x in n.properDivisors, x) ∣ n) :
       have hlt : ∑ x in n.succ.succ.properDivisors, x < n.succ.succ :=
         lt_of_le_of_ne (Nat.le_of_dvd (Nat.succ_pos _) h) ne_n
       symm
-      rw [← mem_singleton,
-        eq_properDivisors_of_subset_of_sum_eq_sum
-          (singleton_subset_iff.2 (mem_properDivisors.2 ⟨h, hlt⟩)) sum_singleton,
-        mem_properDivisors]
+      rw [← mem_singleton, eq_properDivisors_of_subset_of_sum_eq_sum (singleton_subset_iff.2
+        (mem_properDivisors.2 ⟨h, hlt⟩)) (sum_singleton _ _), mem_properDivisors]
       refine' ⟨one_dvd _, Nat.succ_lt_succ (Nat.succ_pos _)⟩
 #align nat.sum_proper_divisors_dvd Nat.sum_properDivisors_dvd
 
@@ -416,13 +414,13 @@ theorem sum_properDivisors_eq_one_iff_prime : ∑ x in n.properDivisors, x = 1 
   · cases n
     · simp [Nat.not_prime_one]
     · rw [← properDivisors_eq_singleton_one_iff_prime]
-      refine' ⟨fun h => _, fun h => h.symm ▸ sum_singleton⟩
+      refine' ⟨fun h => _, fun h => h.symm ▸ sum_singleton _ _⟩
       rw [@eq_comm (Finset ℕ) _ _]
       apply
         eq_properDivisors_of_subset_of_sum_eq_sum
           (singleton_subset_iff.2
             (one_mem_properDivisors_iff_one_lt.2 (succ_lt_succ (Nat.succ_pos _))))
-          (Eq.trans sum_singleton h.symm)
+          ((sum_singleton _ _).trans h.symm)
 #align nat.sum_proper_divisors_eq_one_iff_prime Nat.sum_properDivisors_eq_one_iff_prime
 
 theorem mem_properDivisors_prime_pow {p : ℕ} (pp : p.Prime) (k : ℕ) {x : ℕ} :

Mathlib/Topology/Algebra/InfiniteSum/Order.lean

@@ -88,7 +88,7 @@ theorem isLUB_hasSum (h : ∀ i, 0 ≤ f i) (hf : HasSum f a) :
 
 theorem le_hasSum (hf : HasSum f a) (i : ι) (hb : ∀ j, j ≠ i → 0 ≤ f j) : f i ≤ a :=
   calc
-    f i = ∑ i in {i}, f i := Finset.sum_singleton.symm
+    f i = ∑ i in {i}, f i := by rw [sum_singleton]
     _ ≤ a := sum_le_hasSum _ (by simpa) hf
 #align le_has_sum le_hasSum