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