# plotting an Integration output

How to solve this integral by Mathematica even by numerical methods (plotting the solution)

Integrate[(Cos[x] - a)/(1 + a^2 - 2*a*Cos[x])^1.5, {x, 0, 2*Pi}]


It consumes huge time on Mathematica 9 home edition and no output.

I just want the Taylor expansion in the vicinity of a=0

-
Why integral over [0,1] when you have [0, 2 Pi] in the picture ? – b.gatessucks Oct 23 '13 at 20:24
Yes, it was an error. I edited the post to be [0, 2 Pi]. Thanks – أحمد كمال Oct 24 '13 at 2:08

expr =  Assuming[a > 0, Integrate[(Cos[x] - a)/(1 + a^2 - 2*a*Cos[x])^(3/2), {x, 0, 1}]]


gives:

(-(-1 + a) Sqrt[1 + a^2 - 2 a Cos[1]] ((-1 + a) EllipticE[1/
2, -((4 a)/(-1 + a)^2)] + (1 + a) EllipticF[1/
2, -((4 a)/(-1 + a)^2)]) -
2 a Abs[-1 + a] Sin[1])/(a (-1 + a^2) Abs[-1 + a] Sqrt[
1 + a^2 - 2 a Cos[1]])


Note: I replaced 1.5 by the radical 3/2. If you want numerical value use NIntegrate

As suggested by b.gatessuck (also belisarius' answer), taking the indefinite integral makes this faster:

expr = Integrate[(Cos[x] - a)/(1 + a^2 - 2*a*Cos[x])^(3/2), x]


Gives:

 (-(-1 + a)^2 Sqrt[(1 + a^2 - 2 a Cos[x])/(-1 + a)^2]
EllipticE[x/2, -((4 a)/(-1 + a)^2)] - (-1 + a^2) Sqrt[(
1 + a^2 - 2 a Cos[x])/(-1 + a)^2]
EllipticF[x/2, -((4 a)/(-1 + a)^2)] -
2 a Sin[x])/(a (-1 + a^2) Sqrt[1 + a^2 - 2 a Cos[x]])


You can then Plot this like any other function:

Plot[(expr/.x -> 1) - ( expr /. x -> 0 ), {a, 0, 5}]


OR from 0 to 2 Pi

Plot[(expr/.x -> 2 Pi) - ( expr /. x -> 0 ), {a, 0, 5}]


-
Try the indefinite integral first, then take the limits and see if it's faster. – b.gatessucks Oct 23 '13 at 20:25
@b.gatessucks. Thanks, that definitely makes it instantaneous. – RunnyKine Oct 23 '13 at 20:30
And that was my answer :) – Dr. belisarius Oct 23 '13 at 20:32
@belisarius Beat you by a whole 35 seconds. – b.gatessucks Oct 23 '13 at 20:32
@b.gatessucks We should take some relativistic effects in account. Here G is fluctuating madly right now – Dr. belisarius Oct 23 '13 at 20:34

This is instantaneous:

p = Integrate[(Cos[x] - a)/(1 + a^2 - 2*a*Cos[x])^(3/2), x,
Assumptions -> Element[a, Reals], GenerateConditions -> True];
Plot[(p /. x -> 1) - (p /. x -> 0), {a, 0, 5}]


In case you made a mistake and the upper integration limit was 2 Pi:

Plot[(p /. x -> 2 Pi) - (p /. x -> 0), {a, 0, 5}]


Edit

As for the series expansion:

s = Normal@Series[(p /. x -> 2 Pi) - (p /. x -> 0), {a, 0, 10}]


a π + (9 a^3 π)/8 + (75 a^5 π)/64 + (1225 a^7 π)/1024 + (19845 a^9 π)/16384

The numerators are the Denominators of the column sums of the BG2 matrix

The denominators follows some power of 2 rule, but not trivial:

s = Series[(p /. x -> 2 Pi) - (p /. x -> 0), {a, 0, 100}];
lf = Denominator /@ List @@ Normal@s;
ListLogPlot@lf


Perhaps the following may help you if you need to find a closed expression:

s = Series[(p /. x -> 2 Pi) - (p /. x -> 0), {a, 0, 1000}];
lf = Differences[Last /@ Flatten /@ FactorInteger /@ Denominator /@ List @@ Normal@s];
ListLinePlot@lf


-
Just p = Integrate[(Cos[x] - a)/(1 + a^2 - 2*a*Cos[x])^(3/2), x] will do, other options seem to be redundant in this case. – Rorschach Oct 23 '13 at 20:37
@Blackbird GenerateConditions is a safeguard against special values, so I'd rather prefer to let it be. The Reals assumption is just a clichè – Dr. belisarius Oct 23 '13 at 20:41

This will compute the Taylor expansion:

Define the integrand.

g[θ_, a_] := (Cos[θ] - a)/(1 + a^2 - 2 a Cos[θ])^(3/2)


Integrate a series expansion.

gi[a_, n_] := Integrate[Series[g[θ, a], {a, 0, n}] // Normal, {θ, 0, 2 π}]


Find a sequence function that fits a sufficiently long series expansion.

gs[a_, n_] = FindSequenceFunction[List @@ Expand@gi[a, 15]][n] // FullSimplify

(* (4 (a^2)^n Gamma[1/2 + n]^2)/(a Gamma[n] Gamma[1 + n]) *)


Summing this over $n = 1, 2, ...$ gives you the required series expansion.

-
Nice one :) I tried FindSequenceFunction[] with 12 terms and then abandoned the idea. – Dr. belisarius Oct 24 '13 at 3:09