# Exercise V.2.1 - minor mistake

**URL:** <https://discuss.pennylane.ai/t/exercise-v-2-1-minor-mistake/7272>\
**Category:** Codebook\
**Created:** [October 13, 2024, 5:20pm UTC](https://discuss.pennylane.ai/t/exercise-v-2-1-minor-mistake/7272 "2024-10-13T17:20:46Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![maximus](https://avatars.discourse-cdn.com/v4/letter/m/57b2e6/32.png) [@maximus](https://discuss.pennylane.ai/u/maximus)\
**Post date:** [October 13, 2024, 5:20pm UTC](https://discuss.pennylane.ai/t/exercise-v-2-1-minor-mistake/7272/1 "2024-10-13T17:20:46Z")

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At the start of the solution, it says:

> Let’s rewrite the Hamiltonian as a combination of its eigenstates
> 
> H = \sum\_i E\_i |E\_i\rangle

I think this equation is meant to look like H |E\_i\rangle = E\_i|E\_i\rangle, since the Hamiltonian is an operator not a state. Rest of the solution looks ok.

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<div class="post-metadata">

**Author:** ![CatalinaAlbornoz](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/catalinaalbornoz/32/1196_2.png) [@CatalinaAlbornoz](https://discuss.pennylane.ai/u/CatalinaAlbornoz)\
**Post date:** [October 18, 2024, 3:47am UTC](https://discuss.pennylane.ai/t/exercise-v-2-1-minor-mistake/7272/2 "2024-10-18T03:47:37Z")

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Thanks for pointing it out @maximus ! 🙌