src/HOL/Hahn_Banach/Function_Norm.thy

tuned proofs -- clarified flow of facts wrt. calculation;

(*  Title:      HOL/Hahn_Banach/Function_Norm.thy
    Author:     Gertrud Bauer, TU Munich
*)

header {* The norm of a function *}

theory Function_Norm
imports Normed_Space Function_Order
begin

subsection {* Continuous linear forms*}

text {*
  A linear form @{text f} on a normed vector space @{text "(V, \<parallel>\<cdot>\<parallel>)"}
  is \emph{continuous}, iff it is bounded, i.e.
  \begin{center}
  @{text "\<exists>c \<in> R. \<forall>x \<in> V. \<bar>f x\<bar> \<le> c \<cdot> \<parallel>x\<parallel>"}
  \end{center}
  In our application no other functions than linear forms are
  considered, so we can define continuous linear forms as bounded
  linear forms:
*}

locale continuous = linearform +
  fixes norm :: "_ \<Rightarrow> real"    ("\<parallel>_\<parallel>")
  assumes bounded: "\<exists>c. \<forall>x \<in> V. \<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>"

declare continuous.intro [intro?] continuous_axioms.intro [intro?]

lemma continuousI [intro]:
  fixes norm :: "_ \<Rightarrow> real"  ("\<parallel>_\<parallel>")
  assumes "linearform V f"
  assumes r: "\<And>x. x \<in> V \<Longrightarrow> \<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>"
  shows "continuous V f norm"
proof
  show "linearform V f" by fact
  from r have "\<exists>c. \<forall>x\<in>V. \<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>" by blast
  then show "continuous_axioms V f norm" ..
qed


subsection {* The norm of a linear form *}

text {*
  The least real number @{text c} for which holds
  \begin{center}
  @{text "\<forall>x \<in> V. \<bar>f x\<bar> \<le> c \<cdot> \<parallel>x\<parallel>"}
  \end{center}
  is called the \emph{norm} of @{text f}.

  For non-trivial vector spaces @{text "V \<noteq> {0}"} the norm can be
  defined as
  \begin{center}
  @{text "\<parallel>f\<parallel> = \<sup>x \<noteq> 0. \<bar>f x\<bar> / \<parallel>x\<parallel>"}
  \end{center}

  For the case @{text "V = {0}"} the supremum would be taken from an
  empty set. Since @{text \<real>} is unbounded, there would be no supremum.
  To avoid this situation it must be guaranteed that there is an
  element in this set. This element must be @{text "{} \<ge> 0"} so that
  @{text fn_norm} has the norm properties. Furthermore it does not
  have to change the norm in all other cases, so it must be @{text 0},
  as all other elements are @{text "{} \<ge> 0"}.

  Thus we define the set @{text B} where the supremum is taken from as
  follows:
  \begin{center}
  @{text "{0} \<union> {\<bar>f x\<bar> / \<parallel>x\<parallel>. x \<noteq> 0 \<and> x \<in> F}"}
  \end{center}

  @{text fn_norm} is equal to the supremum of @{text B}, if the
  supremum exists (otherwise it is undefined).
*}

locale fn_norm =
  fixes norm :: "_ \<Rightarrow> real"    ("\<parallel>_\<parallel>")
  fixes B defines "B V f \<equiv> {0} \<union> {\<bar>f x\<bar> / \<parallel>x\<parallel> | x. x \<noteq> 0 \<and> x \<in> V}"
  fixes fn_norm ("\<parallel>_\<parallel>\<hyphen>_" [0, 1000] 999)
  defines "\<parallel>f\<parallel>\<hyphen>V \<equiv> \<Squnion>(B V f)"

locale normed_vectorspace_with_fn_norm = normed_vectorspace + fn_norm

lemma (in fn_norm) B_not_empty [intro]: "0 \<in> B V f"
  by (simp add: B_def)

text {*
  The following lemma states that every continuous linear form on a
  normed space @{text "(V, \<parallel>\<cdot>\<parallel>)"} has a function norm.
*}

lemma (in normed_vectorspace_with_fn_norm) fn_norm_works:
  assumes "continuous V f norm"
  shows "lub (B V f) (\<parallel>f\<parallel>\<hyphen>V)"
proof -
  interpret continuous V f norm by fact
  txt {* The existence of the supremum is shown using the
    completeness of the reals. Completeness means, that every
    non-empty bounded set of reals has a supremum. *}
  have "\<exists>a. lub (B V f) a"
  proof (rule real_complete)
    txt {* First we have to show that @{text B} is non-empty: *}
    have "0 \<in> B V f" ..
    then show "\<exists>x. x \<in> B V f" ..

    txt {* Then we have to show that @{text B} is bounded: *}
    show "\<exists>c. \<forall>y \<in> B V f. y \<le> c"
    proof -
      txt {* We know that @{text f} is bounded by some value @{text c}. *}
      from bounded obtain c where c: "\<forall>x \<in> V. \<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>" ..

      txt {* To prove the thesis, we have to show that there is some
        @{text b}, such that @{text "y \<le> b"} for all @{text "y \<in>
        B"}. Due to the definition of @{text B} there are two cases. *}

      def b \<equiv> "max c 0"
      have "\<forall>y \<in> B V f. y \<le> b"
      proof
        fix y assume y: "y \<in> B V f"
        show "y \<le> b"
        proof cases
          assume "y = 0"
          then show ?thesis unfolding b_def by arith
        next
          txt {* The second case is @{text "y = \<bar>f x\<bar> / \<parallel>x\<parallel>"} for some
            @{text "x \<in> V"} with @{text "x \<noteq> 0"}. *}
          assume "y \<noteq> 0"
          with y obtain x where y_rep: "y = \<bar>f x\<bar> * inverse \<parallel>x\<parallel>"
              and x: "x \<in> V" and neq: "x \<noteq> 0"
            by (auto simp add: B_def divide_inverse)
          from x neq have gt: "0 < \<parallel>x\<parallel>" ..

          txt {* The thesis follows by a short calculation using the
            fact that @{text f} is bounded. *}

          note y_rep
          also have "\<bar>f x\<bar> * inverse \<parallel>x\<parallel> \<le> (c * \<parallel>x\<parallel>) * inverse \<parallel>x\<parallel>"
          proof (rule mult_right_mono)
            from c x show "\<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>" ..
            from gt have "0 < inverse \<parallel>x\<parallel>" 
              by (rule positive_imp_inverse_positive)
            then show "0 \<le> inverse \<parallel>x\<parallel>" by (rule order_less_imp_le)
          qed
          also have "\<dots> = c * (\<parallel>x\<parallel> * inverse \<parallel>x\<parallel>)"
            by (rule Groups.mult_assoc)
          also
          from gt have "\<parallel>x\<parallel> \<noteq> 0" by simp
          then have "\<parallel>x\<parallel> * inverse \<parallel>x\<parallel> = 1" by simp 
          also have "c * 1 \<le> b" by (simp add: b_def)
          finally show "y \<le> b" .
        qed
      qed
      then show ?thesis ..
    qed
  qed
  then show ?thesis unfolding fn_norm_def by (rule the_lubI_ex)
qed

lemma (in normed_vectorspace_with_fn_norm) fn_norm_ub [iff?]:
  assumes "continuous V f norm"
  assumes b: "b \<in> B V f"
  shows "b \<le> \<parallel>f\<parallel>\<hyphen>V"
proof -
  interpret continuous V f norm by fact
  have "lub (B V f) (\<parallel>f\<parallel>\<hyphen>V)"
    using `continuous V f norm` by (rule fn_norm_works)
  from this and b show ?thesis ..
qed

lemma (in normed_vectorspace_with_fn_norm) fn_norm_leastB:
  assumes "continuous V f norm"
  assumes b: "\<And>b. b \<in> B V f \<Longrightarrow> b \<le> y"
  shows "\<parallel>f\<parallel>\<hyphen>V \<le> y"
proof -
  interpret continuous V f norm by fact
  have "lub (B V f) (\<parallel>f\<parallel>\<hyphen>V)"
    using `continuous V f norm` by (rule fn_norm_works)
  from this and b show ?thesis ..
qed

text {* The norm of a continuous function is always @{text "\<ge> 0"}. *}

lemma (in normed_vectorspace_with_fn_norm) fn_norm_ge_zero [iff]:
  assumes "continuous V f norm"
  shows "0 \<le> \<parallel>f\<parallel>\<hyphen>V"
proof -
  interpret continuous V f norm by fact
  txt {* The function norm is defined as the supremum of @{text B}.
    So it is @{text "\<ge> 0"} if all elements in @{text B} are @{text "\<ge>
    0"}, provided the supremum exists and @{text B} is not empty. *}
  have "lub (B V f) (\<parallel>f\<parallel>\<hyphen>V)"
    using `continuous V f norm` by (rule fn_norm_works)
  moreover have "0 \<in> B V f" ..
  ultimately show ?thesis ..
qed

text {*
  \medskip The fundamental property of function norms is:
  \begin{center}
  @{text "\<bar>f x\<bar> \<le> \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>"}
  \end{center}
*}

lemma (in normed_vectorspace_with_fn_norm) fn_norm_le_cong:
  assumes "continuous V f norm" "linearform V f"
  assumes x: "x \<in> V"
  shows "\<bar>f x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>V * \<parallel>x\<parallel>"
proof -
  interpret continuous V f norm by fact
  interpret linearform V f by fact
  show ?thesis
  proof cases
    assume "x = 0"
    then have "\<bar>f x\<bar> = \<bar>f 0\<bar>" by simp
    also have "f 0 = 0" by rule unfold_locales
    also have "\<bar>\<dots>\<bar> = 0" by simp
    also have a: "0 \<le> \<parallel>f\<parallel>\<hyphen>V"
      using `continuous V f norm` by (rule fn_norm_ge_zero)
    from x have "0 \<le> norm x" ..
    with a have "0 \<le> \<parallel>f\<parallel>\<hyphen>V * \<parallel>x\<parallel>" by (simp add: zero_le_mult_iff)
    finally show "\<bar>f x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>V * \<parallel>x\<parallel>" .
  next
    assume "x \<noteq> 0"
    with x have neq: "\<parallel>x\<parallel> \<noteq> 0" by simp
    then have "\<bar>f x\<bar> = (\<bar>f x\<bar> * inverse \<parallel>x\<parallel>) * \<parallel>x\<parallel>" by simp
    also have "\<dots> \<le>  \<parallel>f\<parallel>\<hyphen>V * \<parallel>x\<parallel>"
    proof (rule mult_right_mono)
      from x show "0 \<le> \<parallel>x\<parallel>" ..
      from x and neq have "\<bar>f x\<bar> * inverse \<parallel>x\<parallel> \<in> B V f"
        by (auto simp add: B_def divide_inverse)
      with `continuous V f norm` show "\<bar>f x\<bar> * inverse \<parallel>x\<parallel> \<le> \<parallel>f\<parallel>\<hyphen>V"
        by (rule fn_norm_ub)
    qed
    finally show ?thesis .
  qed
qed

text {*
  \medskip The function norm is the least positive real number for
  which the following inequation holds:
  \begin{center}
    @{text "\<bar>f x\<bar> \<le> c \<cdot> \<parallel>x\<parallel>"}
  \end{center}
*}

lemma (in normed_vectorspace_with_fn_norm) fn_norm_least [intro?]:
  assumes "continuous V f norm"
  assumes ineq: "\<And>x. x \<in> V \<Longrightarrow> \<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>" and ge: "0 \<le> c"
  shows "\<parallel>f\<parallel>\<hyphen>V \<le> c"
proof -
  interpret continuous V f norm by fact
  show ?thesis
  proof (rule fn_norm_leastB [folded B_def fn_norm_def])
    fix b assume b: "b \<in> B V f"
    show "b \<le> c"
    proof cases
      assume "b = 0"
      with ge show ?thesis by simp
    next
      assume "b \<noteq> 0"
      with b obtain x where b_rep: "b = \<bar>f x\<bar> * inverse \<parallel>x\<parallel>"
        and x_neq: "x \<noteq> 0" and x: "x \<in> V"
        by (auto simp add: B_def divide_inverse)
      note b_rep
      also have "\<bar>f x\<bar> * inverse \<parallel>x\<parallel> \<le> (c * \<parallel>x\<parallel>) * inverse \<parallel>x\<parallel>"
      proof (rule mult_right_mono)
        have "0 < \<parallel>x\<parallel>" using x x_neq ..
        then show "0 \<le> inverse \<parallel>x\<parallel>" by simp
        from x show "\<bar>f x\<bar> \<le> c * \<parallel>x\<parallel>" by (rule ineq)
      qed
      also have "\<dots> = c"
      proof -
        from x_neq and x have "\<parallel>x\<parallel> \<noteq> 0" by simp
        then show ?thesis by simp
      qed
      finally show ?thesis .
    qed
  qed (insert `continuous V f norm`, simp_all add: continuous_def)
qed

end