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From the document we know that

Arg[z] gives the gives the argument of the complex number z.

Then how about Arg'[z]? This seems to be meaningless, but Mathematica returns something if z is a non-exact number, for example

Arg'[1. + I]
(* -0.5 *)

So my question is:

  1. How is the numeric value of Arg'[z] defined?

  2. Why does Arg' behave like this? What's the potential usage of this behavior?

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  • $\begingroup$ This can shed some light: Trace[ Arg'[1. + I], TraceInternal -> True ] $\endgroup$
    – Kuba
    Apr 25, 2019 at 8:04
  • $\begingroup$ It is impossible to understand what the output from Trace[ Arg'[1. + I], TraceInternal -> True ] mean. May be numerics gone mad or something :) so just change 1.0 to 1 in the example given and then Arg will no longer do what you show. $\endgroup$
    – Nasser
    Apr 25, 2019 at 8:08
  • $\begingroup$ @Kuba Oh blinding light… $\endgroup$
    – xzczd
    Apr 25, 2019 at 8:09
  • $\begingroup$ @Nasser and xzczd it looks like it calculates derivative value from set of points f or Arg, but I wasn't paying too much attention. $\endgroup$
    – Kuba
    Apr 25, 2019 at 8:11
  • 1
    $\begingroup$ Related: mathematica.stackexchange.com/questions/29329/… $\endgroup$
    – Michael E2
    May 1, 2019 at 15:27

2 Answers 2

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The internal Trace[] Kuba advises shows calculations consistent with the numeric approximation of the partial derivative with respect to the real part:

D[ComplexExpand[Arg[x + I y], TargetFunctions -> {Re, Im}], x] /. 
  x -> 1 /. y -> 1
(*  -(1/2)  *)

This is what Mathematica does with the derivative of a numeric function with approximate input.

Other examples:

ClearAll[f, g];
f[x_?NumericQ] := Re[x]^2;
g[x_?NumericQ] := Im[x]^2;

f'[1. + I]
g'[1. + I]
(*
  1.999999999999995`  
  -2.7506672371246275`*^-15 
*)

It seems like the wrong way to evaluate Derivative.

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  • $\begingroup$ Funny, Abs'[2. + I] returns the input. $\endgroup$
    – xzczd
    Apr 25, 2019 at 11:19
  • $\begingroup$ @xzczd I guess they protected Abs'[] from being evaluated in this way. Re'[] and Im'[] do not evaluate, also. Maybe they overlooked Arg'[]? $\endgroup$
    – Michael E2
    Apr 25, 2019 at 14:21
5
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The definition of the argument is $\arg(z)=\text{Im}(\ln(z))$. Its partial derivative with respect to $z$ would then be

$$ \frac{\partial \arg(z)}{\partial z}= \frac{\partial}{\partial z}\frac{\ln(z)-\ln(z^*)}{2i} = -\frac{i}{2z}. $$

What you see looks like twice the real part of this expression:

With[{z = 2. + 5 I},
  {Arg'[z], 2Re[-I/(2z)]}]

(* {-0.172414, -0.172414} *)

I don't know in which sense this is the "correct" answer. It could be that what is actually calculated is not the partial derivative with respect to $z$, but rather the partial derivative with respect to the real part of $z$:

$$ \frac{\partial \arg(z)}{\partial\text{Re}(z)} =\frac{\partial \arg(z)}{\partial z}\frac{\partial z}{\partial\text{Re}(z)} +\frac{\partial \arg(z)}{\partial z^*}\frac{\partial z^*}{\partial\text{Re}(z)}\\ =\frac{\partial \arg(z)}{\partial z} +\frac{\partial \arg(z)}{\partial z^*} = -\frac{i}{2z}+\frac{i}{2z^*} = -\frac{\text{Im}(z)}{|z|^2} $$

With[{z = 2. + 5 I},
  {Arg'[z], 2Re[-I/(2z)], -Im[z]/Abs[z]^2}]

(* {-0.172414, -0.172414, -0.172414} *)
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  • $\begingroup$ Er… how is derivative of $\ln(z^*)$ defined here? $\endgroup$
    – xzczd
    Apr 25, 2019 at 8:45
  • $\begingroup$ Any idea why then With[{z = 1.0 + I}, Arg'[z]] not same as With[{z = 1 + I}, Arg'[z]]? Should not these give same result? $\endgroup$
    – Nasser
    Apr 25, 2019 at 8:47
  • $\begingroup$ @xzczd $z$ and $z^*$ are independent variables in complex analysis, so $\partial z^*/\partial z=0$ etc. $\endgroup$
    – Roman
    Apr 25, 2019 at 8:48
  • 1
    $\begingroup$ @Nasser they do give the same result when you apply N: Arg'[1 + I] // N also gives -0.5. $\endgroup$
    – Roman
    Apr 25, 2019 at 8:50

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