Inner product space

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An inner product space (AKA an i.p.s or a pre-hilbert space) is a[1]:

  • Vector space (over the field [ilmath]\mathbb{R} [/ilmath] or [ilmath]\mathbb{C} [/ilmath], which we shall denote [ilmath]F[/ilmath]) [ilmath](X,F)[/ilmath], equipped with an
  • Inner product, [ilmath]\langle\cdot,\cdot\rangle[/ilmath]

We denote this [ilmath](X,\langle\cdot,\cdot\rangle,F)[/ilmath] or just [ilmath](X,\langle\cdot,\cdot\rangle)[/ilmath] if the field is implicit.


All i.p.s are also normed spaces as there is an induced norm on an i.p.s given by:

  • For an [ilmath]x\in X[/ilmath] we define [math]\Vert x\Vert:=\sqrt{\langle x,x\rangle^2}[/math]

(which as per the article in turn induces its own metric: [ilmath]d(x,y):=\Vert x-y\Vert[/ilmath])


  1. Functional Analysis - George Bachman and Lawrence Narici