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MathGroup Archive 2006

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Re: sum of integrals over patial intervals != integral over whole interval

  • To: mathgroup at smc.vnet.net
  • Subject: [mg71732] Re: sum of integrals over patial intervals != integral over whole interval
  • From: "dimitris" <dimmechan at yahoo.com>
  • Date: Tue, 28 Nov 2006 06:03:36 -0500 (EST)
  • References: <ekec90$fp$1@smc.vnet.net>

$VersionNumber
5.2

Of course your integral is zero.

f[x_] := Log[Sin[x]^2]*Tan[x]

Plot[f[x], {x, 0, Pi}]

Plus @@ (NIntegrate[f[x], {x, #[[1]], #[[2]]}, WorkingPrecision -> 40,
PrecisionGoal -> 30] & /@ Partition[Range[0, Pi, Pi/2], 2, 1])
0``29.90647836759534

Integrate[f[x], {x, 0, Pi}]
Integrate::idiv: Integral of ` ` does not converge on {x,0,p}.
Integrate[Log[Sin[x]^2]*Tan[x], {x, 0, Pi}]

Some ways to work are:

Timing[Integrate[f[x], {x, 0, Pi/2, Pi}]]
{11.422*Second, 0}

This setting of Integrate seems undocumentated; but it is well
documentated
for NIntegrate.

(see also the following link within MathGroup

http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/thread/fb1a23a6551a4d12/623c24b8307057ce?lnk=gst&q=Integrate+third+argument&rnum=3&hl=en#623c24b8307057ce
 )

Or

Plus @@ (Integrate[f[x], {x, #1[[1]], #1[[2]]}] & ) /@
Partition[Range[0, Pi, Pi/2], 2, 1]
0

Or

F[x_] = Integrate[f[x], x]
Log[Sec[x/2]^2]^2 + 2*Log[Sec[x/2]^2]*Log[(1/2)*Cos[x]*Sec[x/2]^2] +
Log[Sec[x/2]^2]*Log[Sin[x]^2] -
  2*Log[Sec[x/2]^2]*Log[-1 + Tan[x/2]^2] - Log[Sin[x]^2]*Log[-1 +
Tan[x/2]^2] + Log[Tan[x/2]^2]*Log[-1 + Tan[x/2]^2] +
  2*PolyLog[2, (1/2)*Sec[x/2]^2] + PolyLog[2, Cos[x]*Sec[x/2]^2] +
PolyLog[2, -Tan[x/2]^2]

FullSimplify[(Limit[F[x], x -> Pi, Direction -> 1] - Limit[F[x], x ->
Pi/2, Direction -> -1]) +
   (Limit[F[x], x -> Pi/2, Direction -> 1] - Limit[F[x], x -> 0,
Direction -> -1])]
0

Or

Integrate[f[x], {x, 0, z}, Assumptions -> Inequality[Pi/2, Less, z,
LessEqual, Pi]]
-(Pi^2/3) - 2*I*Pi*Log[2] + Log[2]^2 + 4*I*Pi*Log[Sec[z/2]] +
4*Log[-(Cos[z]/(1 + Cos[z]))]*Log[Sec[z/2]] + 4*Log[Sec[z/2]]^2 -
  4*Log[Sec[z/2]]*Log[(-Cos[z])*Sec[z/2]^2] +
4*Log[Sec[z/2]]*Log[Sin[z]] - 2*Log[(-Cos[z])*Sec[z/2]^2]*Log[Sin[z]] +

  2*Log[(-Cos[z])*Sec[z/2]^2]*Log[Tan[z/2]] + 2*PolyLog[2, 1/(1 +
Cos[z])] + PolyLog[2, Cos[z]*Sec[z/2]^2] +
  PolyLog[2, -Tan[z/2]^2]

Limit[%, z -> Pi, Direction -> 1]
0

Or

Assuming[a >= 1, Integrate[f[a*x], {x, 0, Pi}]]
(1/(3*a))*(-Pi^2 - 6*I*Pi*Log[2] + 3*Log[2]^2 + 6*Log[Cos[a*Pi]/(1 +
Cos[a*Pi])]*Log[Sec[(a*Pi)/2]^2] +
   3*Log[Sec[(a*Pi)/2]^2]^2 + 3*Log[Sec[(a*Pi)/2]^2]*Log[Sin[a*Pi]^2] -
6*Log[Sec[(a*Pi)/2]^2]*Log[-1 + Tan[(a*Pi)/2]^2] -
   3*Log[Sin[a*Pi]^2]*Log[-1 + Tan[(a*Pi)/2]^2] +
3*Log[Tan[(a*Pi)/2]^2]*Log[-1 + Tan[(a*Pi)/2]^2] +
   6*PolyLog[2, 1/(1 + Cos[a*Pi])] + 3*PolyLog[2,
Cos[a*Pi]*Sec[(a*Pi)/2]^2] + 3*PolyLog[2, -Tan[(a*Pi)/2]^2])

Limit[%, a -> 1]
0

Integrate[f[x], {x, 0, Pi}, GenerateConditions -> False]
0

At last I found one case of the GenerateConditions->False setting that
it is preferable than the default GenerateConditions->True (apart for
issues of integration in the Hadamard sense
and integrations where you know in advance for what parameters the
integral converges).

Unfortunately, I cannot find a reason to explain why
Integrate[f[x],{x,0,Pi}] fails here.

Even from the following plots:

Show[GraphicsArray[Block[{$DisplayFunction = Identity},
    (ContourPlot[Evaluate[#1[f[z] /. z -> x + I*y]], {x, -Pi, Pi}, {y,
-Pi, Pi}, Contours -> 50, PlotPoints -> 50,
       ContourShading -> False] & ) /@ {Re, Im}]], ImageSize -> 500]

Show[GraphicsArray[Block[{$DisplayFunction = Identity},
    (ContourPlot[Evaluate[#1[F[z] /. z -> x + I*y]], {x, -Pi, Pi}, {y,
-Pi, Pi}, Contours -> 50, PlotPoints -> 50,
       ContourShading -> False] & ) /@ {Re, Im}]], ImageSize -> 500]

Show[GraphicsArray[Block[{$DisplayFunction = Identity},
    (Plot3D[Evaluate[#1[f[z] /. z -> x + I*y]], {x, -Pi, Pi}, {y, -Pi,
Pi}, PlotPoints -> 50] & ) /@ {Re, Im}]],
  ImageSize -> 500]

Show[GraphicsArray[Block[{$DisplayFunction = Identity},
    (Plot3D[Evaluate[#1[F[z] /. z -> x + I*y]], {x, -Pi, Pi}, {y, -Pi,
Pi}, PlotPoints -> 50] & ) /@ {Re, Im}]], ImageSize -> 500]


Regards
Dimitris

Peter Pein wrote:
> Dear group,
>
> I wanted Mathematica to show, that for f[x_]:=Log[Sin[x]^2]Tan[x],
> Integrate[f[x],{x,0,Pi}]==0, because f[x]+f[Pi-x]==0.
>
> Mathematica says Integrate[f[x],{x,0,Pi}] does not converge, but
> Integrate[f[x],{x,0,Pi/2}] and Integrate[f[x],{x,Pi/2,Pi}] evaluate to
> -Pi^2/12 resp. P^2/12 and the sum is zero. The more general integral
> Integrate[f[x],{x,0,z},Assumptions->Pi/2<z<=Pi] evaluates explicitly (?).
>
> What did I do wrong?
> http://people.freenet.de/Peter_Berlin/Mathe/komisch.nb
> 
> TIA,
> Peter


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