title: De Branges space
text: # De Branges space
  
  In mathematics, a de Branges space (sometimes written De Branges space) is a concept in functional analysis and is constructed from a de Branges function.
  
  The concept is named after Louis de Branges who proved numerous results regarding these spaces, especially as Hilbert spaces, and used those results to prove the Bieberbach conjecture.
  
  ## De Branges functions
  
  A Hermite-Biehler function, also known as de Branges function is an entire function E from $\mathbb {C}$ to $\mathbb {C}$ that satisfies the inequality $|E(z)|>|E({\bar {z}})|$, for all z in the upper half of the complex plane $\mathbb {C} ^{+}=\{z\in \mathbb {C} \mid \operatorname {Im} (z)>0\}$.
  
  ## Definition 1
  
  Given a Hermite-Biehler function E, the de Branges space B(E) is defined as the set of all entire functions F such that
  
  $F/E,F^{\#}/E\in H_{2}(\mathbb {C} ^{+})$
  
  where:
  
  * $\mathbb {C} ^{+}=\{z\in \mathbb {C} \mid \operatorname {Im} (z)>0\}$ is the open upper half of the complex plane.
  * $F^{\#}(z)={\overline {F({\bar {z}})}}$.
  * $H_{2}(\mathbb {C} ^{+})$ is the usual Hardy space on the open upper half plane.
  
  ## Definition 2
  
  A de Branges space can also be defined as all entire functions F satisfying all of the following conditions:
  
  * $\int _{\mathbb {R} }|(F/E)(\lambda )|^{2}d\lambda <\infty$
  * $|(F/E)(z)|,|(F^{\#}/E)(z)|\leq C_{F}(\operatorname {Im} (z))^{(-1/2)},\forall z\in \mathbb {C} ^{+}$
  
  ## Definition 3
  
  There exists also an axiomatic description, useful in operator theory.
  
  ## As Hilbert spaces
  
  Given a de Branges space B(E). Define the scalar product:
  
  $[F,G]={\frac {1}{\pi }}\int _{\mathbb {R} }{\overline {F(\lambda )}}G(\lambda ){\frac {d\lambda }{|E(\lambda )|^{2}}}.$
  
  A de Branges space with such a scalar product can be proven to be a Hilbert space.
  
  