## should it be upside or downside

how does the triangle symbol in Helmholtz Equation should be?
is it like the attach file in here.
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 or like this attach file. btw, can you attach more then one file in here?
 If I remember correctly, then they are both the same. nabla squared is sometimes denoted as a triangle with the sharp side up. So: $$\nabla^2 \equiv \Delta$$

Recognitions:
 I also usually see $\nabla^2$. But $\Delta$ is perfectly valid. Personally I prefer $\nabla^2$. It makes it more clear what operation you're actually performing. Plus it help avoid defining a new symbol for every single mathematical operation. And $\Delta$ is already used enough anyway. In the Helmholtz equation in particular $\nabla^2$ looks better. $\nabla^2\psi+k^2\psi=0$ is more symmetric.
 Recognitions: Gold Member Science Advisor Staff Emeritus loop quantum gravity: note well it is $$\nabla^2$$ and $$\Delta$$ that are the same not $$\nabla$$and $$\Delta$$!