Equivalence of Definitions of Hilbert Space

Theorem
Let $H$ be a vector space over $\mathbb F \in \set {\R, \C}$.

Definition 1 implies Definition 2
By definition of complete metric space, every Cauchy sequence in $H$ is convergent.

By definition of Banach space, it follows that $\struct { H, \norm {\,\cdot\,}_H }$ is a Banach space.

Definition 2 implies Definition 1
By definition of inner product space, it follows that $\struct { H, \innerprod \cdot \cdot_H }$ is an inner product space.

Denote the inner product norm as $\norm {\,\cdot\,}_*$.

Let $\sequence {x_n}_{n \mathop \in \N}$ be a Cauchy sequence in $\struct {H, \norm {\,\cdot\,}_H}$.

By definition of Banach space, it follows that $\sequence {x_n}_{n \mathop \in \N}$ is a convergent sequence in $\struct {H, \norm {\,\cdot\,}_H}$.

By Cauchy Sequences in Vector Spaces with Equivalent Norms Coincide, it follows that $\sequence {x_n}_{n \mathop \in \N}$ is a Cauchy sequence in $\struct {H, \norm {\,\cdot\,}_*}$.

By Convergent Sequences in Vector Spaces with Equivalent Norms Coincide, it follows that $\sequence {x_n}_{n \mathop \in \N}$ is a convergent sequence in $\struct {H, \norm {\,\cdot\,}_*}$.

Let $d: H \times H \to \R_{\ge 0}$ be the metric induced by the inner product norm $\norm {\,\cdot\,}_*$.

By definition of complete metric space, it follows that $\struct {H, d}$ is a complete metric space.