More explanation about the integral.

Variational/Variational_Hydrogen.ipynb

@@ -146,7 +146,7 @@
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    "metadata": {},
    "source": [
-    "Construct the integral in spherical coordinates  (There should be an additional factor of $4 \\pi$, but it will cancel since it occurs in the numerator and denominator)"
+    "The integration occurs in 3D over the electron coordinates.  Because the integrand only has a dependence on $r$, it can be converted to spherical coordinates, and reduced to a 1D integral over $r$. (There should be an additional factor of $4 \\pi$, but it will cancel since it occurs in the numerator and denominator)"
    ]
   },
   {