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The very first thing my textbook says is that the Del operator is defined as: $$\vec{\nabla}=\vec{a}^i\nabla_i$$ Where $\nabla_i$ is the covariant derivative and " $\vec{a}^i$ is the curvilinear basis of the curvilinear coordinates $\xi ^i$". However I can't understand if that should be the dual basis (because of the upper index) or it is just written that way to obey einstein notation (upper and lower repeating) or maybe it is a mistake (I don't believe it to be). Can you help me clarify this?

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    $\begingroup$ What textbook?? $\endgroup$
    – DanielC
    Commented Oct 21, 2023 at 15:35
  • $\begingroup$ Is this supposed to be the Hamiltonian operator in quantum mechanics? $\endgroup$ Commented Oct 21, 2023 at 15:44
  • $\begingroup$ Have you explored the gradient in spherical coordinates? $\endgroup$ Commented Oct 21, 2023 at 16:01
  • $\begingroup$ DanielC "Лекции по векторно и тензорно смятане за физици" It's in Bulgarian $\endgroup$ Commented Oct 21, 2023 at 16:15
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    $\begingroup$ I've never heard of it being called a Hamilton operator. Me neither. $\endgroup$
    – Ghoster
    Commented Oct 21, 2023 at 17:27

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