Theory Indicator_Function

(*  Title:      HOL/Library/Indicator_Function.thy
    Author:     Johannes Hoelzl (TU Muenchen)
*)

section ‹Indicator Function›

theory Indicator_Function
imports Complex_Main Disjoint_Sets
begin

definition "indicator S x = (if x ∈ S then 1 else 0)"

text‹Type constrained version›
abbreviation indicat_real :: "'a set ⇒ 'a ⇒ real" where "indicat_real S ≡ indicator S"

lemma indicator_simps[simp]:
  "x ∈ S ⟹ indicator S x = 1"
  "x ∉ S ⟹ indicator S x = 0"
  unfolding indicator_def by auto

lemma indicator_pos_le[intro, simp]: "(0::'a::linordered_semidom) ≤ indicator S x"
  and indicator_le_1[intro, simp]: "indicator S x ≤ (1::'a::linordered_semidom)"
  unfolding indicator_def by auto

lemma indicator_abs_le_1: "¦indicator S x¦ ≤ (1::'a::linordered_idom)"
  unfolding indicator_def by auto

lemma indicator_eq_0_iff: "indicator A x = (0::'a::zero_neq_one) ⟷ x ∉ A"
  by (auto simp: indicator_def)

lemma indicator_eq_1_iff: "indicator A x = (1::'a::zero_neq_one) ⟷ x ∈ A"
  by (auto simp: indicator_def)

lemma indicator_UNIV [simp]: "indicator UNIV = (λx. 1)"
  by auto

lemma indicator_leI:
  "(x ∈ A ⟹ y ∈ B) ⟹ (indicator A x :: 'a::linordered_nonzero_semiring) ≤ indicator B y"
  by (auto simp: indicator_def)

lemma split_indicator: "P (indicator S x) ⟷ ((x ∈ S ⟶ P 1) ∧ (x ∉ S ⟶ P 0))"
  unfolding indicator_def by auto

lemma split_indicator_asm: "P (indicator S x) ⟷ (¬ (x ∈ S ∧ ¬ P 1 ∨ x ∉ S ∧ ¬ P 0))"
  unfolding indicator_def by auto

lemma indicator_inter_arith: "indicator (A ∩ B) x = indicator A x * (indicator B x::'a::semiring_1)"
  unfolding indicator_def by (auto simp: min_def max_def)

lemma indicator_union_arith:
  "indicator (A ∪ B) x = indicator A x + indicator B x - indicator A x * (indicator B x :: 'a::ring_1)"
  unfolding indicator_def by (auto simp: min_def max_def)

lemma indicator_inter_min: "indicator (A ∩ B) x = min (indicator A x) (indicator B x::'a::linordered_semidom)"
  and indicator_union_max: "indicator (A ∪ B) x = max (indicator A x) (indicator B x::'a::linordered_semidom)"
  unfolding indicator_def by (auto simp: min_def max_def)

lemma indicator_disj_union:
  "A ∩ B = {} ⟹ indicator (A ∪ B) x = (indicator A x + indicator B x :: 'a::linordered_semidom)"
  by (auto split: split_indicator)

lemma indicator_compl: "indicator (- A) x = 1 - (indicator A x :: 'a::ring_1)"
  and indicator_diff: "indicator (A - B) x = indicator A x * (1 - indicator B x ::'a::ring_1)"
  unfolding indicator_def by (auto simp: min_def max_def)

lemma indicator_times:
  "indicator (A × B) x = indicator A (fst x) * (indicator B (snd x) :: 'a::semiring_1)"
  unfolding indicator_def by (cases x) auto

lemma indicator_sum:
  "indicator (A <+> B) x = (case x of Inl x ⇒ indicator A x | Inr x ⇒ indicator B x)"
  unfolding indicator_def by (cases x) auto

lemma indicator_image: "inj f ⟹ indicator (f ` X) (f x) = (indicator X x::_::zero_neq_one)"
  by (auto simp: indicator_def inj_def)

lemma indicator_vimage: "indicator (f -` A) x = indicator A (f x)"
  by (auto split: split_indicator)

lemma  (* FIXME unnamed!? *)
  fixes f :: "'a ⇒ 'b::semiring_1"
  assumes "finite A"
  shows sum_mult_indicator[simp]: "(∑x ∈ A. f x * indicator B x) = (∑x ∈ A ∩ B. f x)"
    and sum_indicator_mult[simp]: "(∑x ∈ A. indicator B x * f x) = (∑x ∈ A ∩ B. f x)"
  unfolding indicator_def
  using assms by (auto intro!: sum.mono_neutral_cong_right split: if_split_asm)

lemma sum_indicator_eq_card:
  assumes "finite A"
  shows "(∑x ∈ A. indicator B x) = card (A Int B)"
  using sum_mult_indicator [OF assms, of "λx. 1::nat"]
  unfolding card_eq_sum by simp

lemma sum_indicator_scaleR[simp]:
  "finite A ⟹
    (∑x ∈ A. indicator (B x) (g x) *⇩R f x) = (∑x ∈ {x∈A. g x ∈ B x}. f x :: 'a::real_vector)"
  by (auto intro!: sum.mono_neutral_cong_right split: if_split_asm simp: indicator_def)

lemma LIMSEQ_indicator_incseq:
  assumes "incseq A"
  shows "(λi. indicator (A i) x :: 'a::{topological_space,one,zero}) ⇢ indicator (⋃i. A i) x"
proof (cases "∃i. x ∈ A i")
  case True
  then obtain i where "x ∈ A i"
    by auto
  then have *:
    "⋀n. (indicator (A (n + i)) x :: 'a) = 1"
    "(indicator (⋃i. A i) x :: 'a) = 1"
    using incseqD[OF ‹incseq A›, of i "n + i" for n] ‹x ∈ A i› by (auto simp: indicator_def)
  show ?thesis
    by (rule LIMSEQ_offset[of _ i]) (use * in simp)
next
  case False
  then show ?thesis by (simp add: indicator_def)
qed

lemma LIMSEQ_indicator_UN:
  "(λk. indicator (⋃i<k. A i) x :: 'a::{topological_space,one,zero}) ⇢ indicator (⋃i. A i) x"
proof -
  have "(λk. indicator (⋃i<k. A i) x::'a) ⇢ indicator (⋃k. ⋃i<k. A i) x"
    by (intro LIMSEQ_indicator_incseq) (auto simp: incseq_def intro: less_le_trans)
  also have "(⋃k. ⋃i<k. A i) = (⋃i. A i)"
    by auto
  finally show ?thesis .
qed

lemma LIMSEQ_indicator_decseq:
  assumes "decseq A"
  shows "(λi. indicator (A i) x :: 'a::{topological_space,one,zero}) ⇢ indicator (⋂i. A i) x"
proof (cases "∃i. x ∉ A i")
  case True
  then obtain i where "x ∉ A i"
    by auto
  then have *:
    "⋀n. (indicator (A (n + i)) x :: 'a) = 0"
    "(indicator (⋂i. A i) x :: 'a) = 0"
    using decseqD[OF ‹decseq A›, of i "n + i" for n] ‹x ∉ A i› by (auto simp: indicator_def)
  show ?thesis
    by (rule LIMSEQ_offset[of _ i]) (use * in simp)
next
  case False
  then show ?thesis by (simp add: indicator_def)
qed

lemma LIMSEQ_indicator_INT:
  "(λk. indicator (⋂i<k. A i) x :: 'a::{topological_space,one,zero}) ⇢ indicator (⋂i. A i) x"
proof -
  have "(λk. indicator (⋂i<k. A i) x::'a) ⇢ indicator (⋂k. ⋂i<k. A i) x"
    by (intro LIMSEQ_indicator_decseq) (auto simp: decseq_def intro: less_le_trans)
  also have "(⋂k. ⋂i<k. A i) = (⋂i. A i)"
    by auto
  finally show ?thesis .
qed

lemma indicator_add:
  "A ∩ B = {} ⟹ (indicator A x::_::monoid_add) + indicator B x = indicator (A ∪ B) x"
  unfolding indicator_def by auto

lemma of_real_indicator: "of_real (indicator A x) = indicator A x"
  by (simp split: split_indicator)

lemma real_of_nat_indicator: "real (indicator A x :: nat) = indicator A x"
  by (simp split: split_indicator)

lemma abs_indicator: "¦indicator A x :: 'a::linordered_idom¦ = indicator A x"
  by (simp split: split_indicator)

lemma mult_indicator_subset:
  "A ⊆ B ⟹ indicator A x * indicator B x = (indicator A x :: 'a::comm_semiring_1)"
  by (auto split: split_indicator simp: fun_eq_iff)

lemma indicator_sums:
  assumes "⋀i j. i ≠ j ⟹ A i ∩ A j = {}"
  shows "(λi. indicator (A i) x::real) sums indicator (⋃i. A i) x"
proof (cases "∃i. x ∈ A i")
  case True
  then obtain i where i: "x ∈ A i" ..
  with assms have "(λi. indicator (A i) x::real) sums (∑i∈{i}. indicator (A i) x)"
    by (intro sums_finite) (auto split: split_indicator)
  also have "(∑i∈{i}. indicator (A i) x) = indicator (⋃i. A i) x"
    using i by (auto split: split_indicator)
  finally show ?thesis .
next
  case False
  then show ?thesis by simp
qed

text ‹
  The indicator function of the union of a disjoint family of sets is the
  sum over all the individual indicators.
›

lemma indicator_UN_disjoint:
  "finite A ⟹ disjoint_family_on f A ⟹ indicator (UNION A f) x = (∑y∈A. indicator (f y) x)"
  by (induct A rule: finite_induct)
    (auto simp: disjoint_family_on_def indicator_def split: if_splits)

end