The integrand has two singular points:
Solve[ 4z^2 + 4z + 3 == 0, z]
> {{z -> 1/2 (-1 - I Sqrt[2])}, {z -> 1/2 (-1 + I Sqrt[2])}}
At infinity it becomes zero:
Limit[ 1/Sqrt[ 4 z^2 + 4 z + 2], z -> ComplexInfinity]
> 0
All these points are the branch points, thus we should define appropriately integration contours in order to avoid possible jumps of the function when moving around a given closed path.
To get a general view on the issue we'll discuss two choices of contours (there are possible many other).
_Mathematica_ arbitrary assumes the branch cuts of the function, it can be easily seen from e.g. `ContourPlot` or `Plot3D` of the real and imaginary parts of the integrand.
On the other hand using the Cauchy Integral Theorem we can choose appropriate contours to perform needed calculations. The main problem here is providing a clear graphical presentation of chosen contours.
##Solution 1
Let's defnie a function which will be used to draw contours:
cp[{x_, y_}, r_, t_, ϕ_, δ_] :=
ConditionalExpression[ {x, y} + r {Cos[t + ϕ], Sin[t + ϕ]}, δ <= t <= 2 Pi - δ]
now supplementing the following plot with arrows and symbols `Style[Subscript[C, #], 28, Bold, FontFamily -> "Times", Blue] & /@ Range[8]` pasted into the graphics with drawing tools (`Ctrl + D`):
Module[
{z1 = 1/2 (-1 - I Sqrt[2]), z2 = 1/2 (-1 + I Sqrt[2]), z11, z22, r = 1/10, δ, δ1},
{z11, z22} = {Re @ #, Im @ #}& /@ {z1, z2}; δ = 2 r; δ1 = δ/7;
ParametricPlot[{ cp[{0, 0}, 1, t, {0}, δ1], cp[ z11, r, t, -Pi + Arg[z1], δ],
cp[ z22, r, t, -Pi + Arg[z2], δ]}, {t, 0, 2 Pi},
PlotStyle -> ConstantArray[{Thick, Darker @ Blue}, 3],
AxesStyle -> Arrowheads[0.03],
PlotRange -> {{-1.05, 1.35}, {-1.1, 1.1}},
Epilog -> {Darker @ Blue, Thick, Line @ {
{ cp[z11, r, δ, -Pi + Arg[z1], 0], { Sin[Arg[z1]] δ1, 0},
cp[z22, r, 2 Pi - δ, -Pi + Arg[z2], 0]},
{ cp[z11, r, 2 Pi - δ, -Pi + Arg[z1], 0], {0, Sin[Arg[z1]] δ1},
{1, Sin[Arg[z1]] δ1}},
{ cp[z22, r, δ, -Pi + Arg[z2], 0], {0, - Sin[Arg[z1]] δ1},
{1, - Sin[Arg[z1]] δ1}}},
Darker @ Magenta, Dashing[{0.035, 0.013}], Thickness[0.004],
Line @ { {z11, {0, 0}, {1.25, 0}}, {z22, {0, 0}}},
Red, PointSize[0.015], Point[{z11, z22}]}, ImageSize -> 750]]
![enter image description here][1]
we can denote integrals over contours $\;C, C_1,\dots,C_8\;$ by `iC, iC1, iC2, ..., iC8`.
From the Cauchy theorem we have:
iC + iC1 + iC2 + iC3 + iC4 + iC5 + iC6 + iC7 + iC8 == 0
for any `r > 0` being the radius of the small circles and `δ` being the half distance between apprporiate parallel contours. On the graphics `δ` and `r` are related, but mathematically we need only evaluating integrals when `r -> 0` and `δ -> 0`. Let's find limits of the integrals `iC3, iC6` when `r` tends to zero. Parametrizing `z` first with `z == z[t] -> z1 + r E^(I t)` and then with `z == z[t] -> z2 + r E^(I t)` we have:
With[{z1 = 1/2 (-1 - I Sqrt[2]), z2 = 1/2 (-1 + I Sqrt[2])},
Limit[{ Integrate[(r I E^(I t))/(2 Sqrt[r E^(I t) (r E^(I t) + z1 - z2)]), {t, 0, 2 Pi},
Assumptions -> r > 0],
Integrate[(r I E^(I t))/(2 Sqrt[r E^(I t) (r E^(I t) + z2 - z1)]), {t, 0, 2 Pi},
Assumptions -> r > 0]}, r -> 0]]
> { 0, 0}
To calculate another integrals we need to observe that after moving around the small contours $C_3$ and $C_6$, e.g.
1/Sqrt[4 z^2 + 4 z + 3] /. z -> 1/2 (-1 - I Sqrt[2]) + r E^(I t) // FullSimplify
> 1/(2 Sqrt[E^(I t) r (-I Sqrt[2] + E^(I t) r)])
the phase changes according to rule: `t -> t + 2Pi`, therefore the integrand has to be multiplied by `-1`. Moreover the both integrals `iC2` and `iC4` as well as `iC5` and `iC7` are calculated with opposite directions thus `iC2 + iC4 == 2 iC2` and `iC5 + iC7 == 2 iC7`. However `iC1 == - iC8` because the integrand changed the sign two times. We need to parametrize `z` on $C_2$ and `C_4` with `z == z[t] -> t z1` and with `z == z[t] -> t z2` on $C_5$ and $C_7$ respectively.
Concluding all of the above remarks we find that:
FullSimplify[
Plus @@ With[{z1 = 1/2 (-1 - I Sqrt[2]), z2 = 1/2 (-1 + I Sqrt[2])},
{ 2 Integrate[ z2/Sqrt[ 4 (t - 1) z2 (t z2 - z1)], {t, 0, 1}],
-2 Integrate[ z1/Sqrt[ 4 (t - 1) z1 (t z1 - z2)], {t, 0, 1}]}]]
> I Pi
thus `iC == -I Pi`.
##Solution 2
Module[
{z1 = 1/2 (-1 - I Sqrt[2]), z2 = 1/2 (-1 + I Sqrt[2]), z11, z22, r = 1/10, δ, δ1},
{z11,z22} = {Re @ #, Im @ #}& /@ {z1, z2}; δ = 2 r; δ1 = δ/7;
ParametricPlot[{cp[{0, 0}, 1, t, Pi, δ1], cp[z11, r, t, Pi/2, δ],
cp[z22, r, t, -Pi/2, δ]}, {t, 0, 2 Pi},
PlotStyle -> ConstantArray[{Thick, Darker@Blue}, 3],
AxesStyle -> Arrowheads[0.03], PlotRange -> {{-1.25, 1.1}, {-1.1, 1.1}},
Epilog -> {Darker @ Blue, Thick, Line @ {
{ cp[z11, r, Pi/2 -δ, 0, 0], cp[z22, r, δ, -Pi/2, 0]},
{ cp[z11, r, Pi/2 + δ], 0, 0], {-1/2 - Sin[δ1], -δ1}, {-1, -δ1}},
{ cp[z22, r, Pi/2 - δ, Pi, 0], {-1/2 - Sin[δ1], δ1}, {-1, δ1}}},
Darker @ Magenta, Dashing[{0.035, 0.013}], Thickness[0.004],
Line @ {{z11, z22}, {{-1.2, 0}, {-1/2, 0}}}, Red, PointSize[0.015],
Point[{z11, z22}]}, ImageSize -> 750]]
![enter image description here][2]
Now `iC1 + iC7 == 0` and `iC2 + iC6 == iC4`. `iC3` and `iC5` tends to zero as `r -> 0`, thus we have:
With[{z1 = 1/2 (-1 - I Sqrt[2]), z2 = 1/2 (-1 + I Sqrt[2])},
Integrate[( 2 I)/Sqrt[3 + 4 (I t + z1) + 4 (I t + z1)^2], {t, 0, Sqrt[2]}]]
> I Pi
therefore `iC == -I Pi`.
We provided the both solutions in the case when the contours are connected, but it is not necessary, it is needed only that they are connected with `ComplexInfinity`.
[1]: https://i.sstatic.net/n1s97.gif
[2]: https://i.sstatic.net/Jt9l6.gif