The ``\nabla'' symbol showed incorrectly as a black disc - latex

Question

I am working on a report, which contains an equation involved gradient. So I need the nabla symbol to be printed out like this:
But actually, I finally got a black disc as I try to input the nabla symbol:
I don't know if it is the problem of my code:
$\nabla_{w}\mathcal{L}(w, b, a)$
As for background details, I use TeXShop4.38 on macOS 10.14.6, the packages are the following:
\usepackage{algorithm}
\usepackage{algpseudocode}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{txfonts}
\usepackage{fdsymbol}
\usepackage{graphicx}
\graphicspath{{secret}}
\DeclareMathOperator*{\argmax}{argmax}
\DeclareMathOperator*{\argsup}{argsup}
Can anyone help me target the problem? Thanks a lot!

Answer 1

You can find the root cause of such problems by creating a minimal example, i.e. the minimum amount of code that is needed to reproduce the problem. In your case that would be:
\documentclass{article}
\usepackage{fdsymbol}
\begin{document}
$\nabla_{w}\mathcal{L}(w, b, a)$
\end{document}
Indeed, the documentation for fdsymbol states that it does not contain a symbol for \nabla. One simple solution would be loading txfonts after fdsymbol. Alternatively, you could update to the more modern newtxtext and newtxmath:
\documentclass{article}
\usepackage[full]{textcomp}
\usepackage{fdsymbol}
\usepackage{newtxtext,newtxmath}
\begin{document}
$\nabla_{w}\mathcal{L}(w, b, a)$
\end{document}
Result:
Notes:
textcomp is needed because both dfdsymbol and newtxtext load is with different options.
You have to decide yourself if fdsymbol is still needed in this case.