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Context

While answering this question, I defined (symbolic and numerical) path integrations as follows

 ContourIntegrate[f_, par : (z_ -> g_), {t_, a_, b_}] := 
       Integrate[Evaluate[(f /. par) D[g, t]], {t, a, b}]

 NContourIntegrate[f_, par : (z_ -> g_), {t_, a_, b_}] := 
       NIntegrate[Evaluate[D[g, t] (f /. par) /. t -> t1], {t1, a, b}]

I also defined a piecewise contour

Clear[pw]; 
pw[t_, a_: 1] = Piecewise[{{a Exp[I t], t < Pi}, {-a + 2 a (t - Pi)/Pi, t >= Pi}}]
ParametricPlot[pw[t] // {Re[#], Im[#]} &, {t, 0, 2 Pi}]

Mathematica graphics

While checking these routines on wikipedia examples, I tried numerically

 Table[NContourIntegrate[Exp[I i x]/(x^2 + 1), x -> pw[t, 2], {t, 0, 2 Pi}], {i, 2, 5}] // Chop

(* {0.425168,0.156411,0.0575403,0.0211679} *)

which corresponds accurately to (see example II for Cauchy distributions)

Table[Exp[-i] Pi, {i, 2, 5}] // N

On the other hand, the symbolic integration

ContourIntegrate[Exp[I x]/(x^2 + 1), x -> pw[t, 2], {t, 0, 2 Pi}] // FullSimplify

returns 0.

Question

What am I doing wrong?

Attempts

Example I and III work ;-)

ContourIntegrate[1/(x^2 + 1)^2, x -> pw[ t, 2], {t, 0, 2 Pi}] // FullSimplify
NContourIntegrate[1/(x^2 + 1)^2, x -> pw[ t, 2], {t, 0, 2 Pi}] // Chop

(* Pi/2  1.5708 *)

and

NContourIntegrate[1/I/x/(1 + 3 ((x + 1/x)/2)^2), x -> Exp[I t], {t, 0, 2 Pi}]//Chop
ContourIntegrate[1/I/x/(1 + 3 ((x + 1/x)/2)^2), x -> Exp[I t], {t, 0, 2 Pi}]

(* 3.14159  Pi *)
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1  
For Example II, with exactly the same code as you have (except that I changed your x more suggestively to z), Mathematica 8.0.4 is giving me result Pi/E. This agrees with the answer provided by the Residue Theorem. –  murray Oct 27 '12 at 15:39
    
@murray uhm... it seems you are right.. –  chris Oct 27 '12 at 15:43
    
The answer you got from ContourIntegrate in Example IIIa is correct -- and is the same, except for form, as the answer $\pi \sqrt{2}/4$ obtained from substitution and the Residue Theorem as in the wikipedia page you cite. –  murray Oct 27 '12 at 15:43
    
@murray right again... ghosh I guess I am out of my mind.. –  chris Oct 27 '12 at 15:48
2  
But you have a nicely defined contour integration routine. Note that it's certainly not necessary to pack parameterization of the entire contour into a single Piecewise function; that just forces one do some contortions to get the t-value at the end of one piece to match up with the t-value at the start of the second piece. It seems it would be simpler to separately parameterize the semicircle and the $x$-axis and evaluate the contour integrals of both; then just add. –  murray Oct 27 '12 at 15:54

1 Answer 1

up vote 6 down vote accepted

The problem lies with the fact that my init.m file has

 SetOptions[Integrate, GenerateConditions -> False]

If I use

 SetOptions[Integrate, GenerateConditions -> True]

or define

 ContourIntegrate[f_, par : (z_ -> g_), {t_, a_, b_}] := 
   Integrate[Evaluate[(f /. par) D[g, t]], {t, a, b}, GenerateConditions -> True]

the discrepency vanishes.

I guess this makes this question too narrow to be of general interest!

In any case, the bring home message is don't do complex integration without paying attention to branch-cuts !

share|improve this answer
1  
No, I don't think this is too narrow an issue: the post might help somebody else with a mysterious result! –  murray Oct 27 '12 at 19:22

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