       Re: fundamental Integrate question

• To: mathgroup at smc.vnet.net
• Subject: [mg73048] Re: [mg73031] fundamental Integrate question
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Wed, 31 Jan 2007 00:04:41 -0500 (EST)
• References: <200701301144.GAA14308@smc.vnet.net>

```dimitris wrote:
> Consider the classical example that incorrectly gave zero in a prior
> version of Mathematica
>
> In:=
> Integrate[f[z], {z, 1 + I, -1 + I, -1 - I, 1 - I, 1 + I}]
> Chop[N[%]]
> Chop[NIntegrate[f[z], {z, 1 + I, -1 + I, -1 - I, 1 - I, 1 + I}]]
>
> Out=
> 2*I*Pi
>
> Out=
> 6.283185307179586*I
>
> Out=
> 6.2831853071795685*I
>
> Of course the result is correct considering the pole at origin and the
> Residue theorem.
>
> Trying to understand how Mathematica applies the Newton-Leibniz
> formula I just want to know if
> I am right below:
>
> In:=
> ((F[z] /. z -> 1 - I) - F[z] /. z -> -1 - I) + ((F[z] /. z -> 1 + I) -
> F[z] /. z -> 1 - I) +
>   ((F[z] /. z -> -1 + I) - F[z] /. z -> 1 + I) + (Limit[F[z], z -> -1,
> Direction -> -I] - F[z] /. z -> -1 + I) +
>   ((F[z] /. z -> -1 - I) - Limit[F[z], z -> -1, Direction -> I])
>
> Out=
> 2*I*Pi
>
> Thanks for any response!
>
> Dimitris

Not clear what you use for f[x]. Maybe 1/x? Anyway, quoting the author
of that notebook:

"It is important to assess whether an integration path crosses a branch
cut of an antiderivative (so that we might split the path into
segments)." So yes, I would imagine it does something along the lines
you suggest.

To see explicitly what limits get computed you might first do

Unprotect[Limit];
Limit[a___] := Null /; (Print[InputForm[limit[a]]]; False)

Daniel Lichtblau
Wolfram Research

```

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