Ivan Shishkin, Rye (1878)

Problems/Real analysisReviewed

An unfamiliar inner product

by Ancient Tree·
20
Difficulty scaleÉchelle de difficulté

This score reflects both the level of the required concepts and the difficulty of the solution.Ce score tient compte à la fois du niveau des notions nécessaires et de la difficulté de la résolution.

  1. 110First steps / middle schoolPremiers pas / collège
  2. 1125Beginner / high schoolDébutant / lycée
  3. 2650Intermediate / undergraduateIntermédiaire / licence
  4. 5170Advanced / graduateAvancé / master
  5. 7190Expert / specializedExpert / spécialisé
  6. 91100Research levelNiveau recherche
These levels are approximate guides.Ces niveaux sont des repères approximatifs.
·
English
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Consider E=F(R)E=\mathcal{F}(\R) the space of real functions f:RRf:\R \rightarrow \R.

Let’s define :
f,g=fg\langle f, g\rangle=f gIs ,\langle,\rangle an inner product ?

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Solution by Ancient Tree

Discussions1 useful vote

This is a trick question. All the properties of an inner product seem to be satisfied : it is bilinear, symmetric, and seemingly positive definite. However, an inner product is with values in R\R, whereas f,g\langle f,g \rangle is a function.

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