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I would like to know how to pronounce in English this symbol: $\nabla \phi$.

It is something phi ... ?

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    $\begingroup$ del phi? $\endgroup$ – Daniel R May 12 '14 at 11:51
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    $\begingroup$ If you know the context, you can also say "grad phi" (if in the context of gradient), or it could be used in the context of curl or divergence. That's a more "informed" pronunciation that tells you more than just plain notation. $\endgroup$ – orion May 12 '14 at 11:56
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    $\begingroup$ Thanks to @Daniel, now I know what to call oracles in the future! $\endgroup$ – Hurkyl May 12 '14 at 12:08
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    $\begingroup$ Detexify can sometimes help with this kind of question: detexify.kirelabs.org/classify.html $\endgroup$ – Phira May 12 '14 at 12:30
  • $\begingroup$ @Phira What an awesome site! That should be stickied somewhere (and probably is already) $\endgroup$ – Mathias711 May 12 '14 at 12:39
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The inversed delta is called Nabla. So Nabla phi. See also http://en.wikipedia.org/wiki/Nabla_symbol

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  • $\begingroup$ Welcome, good question! $\endgroup$ – Nicky Hekster May 12 '14 at 11:53
  • $\begingroup$ Is it also called nabla when dealing with finite differences? I have always called it "nabla" in vector analysis, but I have a doubt for other uses. $\endgroup$ – Jean-Claude Arbaut May 12 '14 at 14:03
  • $\begingroup$ @Jean-Claude Arbaut: nabla is a name of “∇” symbol (cf. equals sign – “=”, times sign – “×”). Del is a name of the operator in vector analysis. The thing with finite differences is not del, but if the symbol is “∇”, then it’s nabla. $\endgroup$ – Incnis Mrsi Dec 13 '14 at 10:34
  • $\begingroup$ @IncnisMrsi This makes sense. In physics, though, we always spelled it "nabla phi", not "del phi", but it may be a french usage. Thank you anyway. $\endgroup$ – Jean-Claude Arbaut Dec 13 '14 at 12:11
  • $\begingroup$ FWIW, "nabla" is an ancient Hebrew word for "harp" or "lyre". Also, if you google it while at work, google may decide that you meant "NAMBLA" and then your boss will want to talk to you. $\endgroup$ – B. Goddard Sep 6 '16 at 23:16
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it's actually del operator, https://en.wikipedia.org/wiki/Del so it is del phi, in mathematics. nabla article points to that for mathematical usage https://en.wikipedia.org/wiki/Nabla_symbol

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