Re: Can't integrate sqrt(a+b*cos(t)+c*cos(2t))
- To: mathgroup at smc.vnet.net
- Subject: [mg90794] Re: Can't integrate sqrt(a+b*cos(t)+c*cos(2t))
- From: "David W.Cantrell" <DWCantrell at sigmaxi.net>
- Date: Thu, 24 Jul 2008 04:54:18 -0400 (EDT)
- References: <g6710s$sb6$1@smc.vnet.net>
Valeri Astanoff <astanoff at gmail.com> wrote: > Good day, > > Neither Mathematica 6 nor anyone here can integrate this: > > In[1]:= Integrate[Sqrt[5 - 4*Cos[t] + Cos[2*t]], {t, 0, Pi}] > Out[1]= Integrate[Sqrt[5 - 4*Cos[t] + Cos[2*t]], {t, 0, Pi}] Actually, we can use Mathematica 6 to integrate that. In[9]:= indef = Integrate[Sqrt[5 - 4*Cos[t] + Cos[2*t]], t] Out[9]= ((2/5 + (4*I)/5)*Sqrt[5 - 4*Cos[t] + Cos[2*t]]*((2 + I)*Sqrt[1 - 2*I]* EllipticE[I*ArcSinh[Sqrt[1 - 2*I]*Tan[t/2]], -(3/5) + (4*I)/5]*(1 + Tan[t/2]^2)*Sqrt[1 + (1 - 2*I)*Tan[t/2]^2]*Sqrt[1 + (1 + 2*I)*Tan[t/2]^2] - I*((6 - 2*I)*Sqrt[1 - 2*I]*EllipticF[I*ArcSinh[Sqrt[1 - 2*I]*Tan[t/2]], -(3/5) + (4*I)/5]*(1 + Tan[t/2]^2)*Sqrt[1 + (1 - 2*I)*Tan[t/2]^2]*Sqrt[1 + (1 + 2*I)*Tan[t/2]^2] - 4*Sqrt[1 - 2*I]*EllipticPi[1/5 + (2*I)/5, I*ArcSinh[Sqrt[1 - 2*I]*Tan[t/2]], -(3/5) + (4*I)/5]*(1 + Tan[t/2]^2)*Sqrt[1 + (1 - 2*I)*Tan[t/2]^2]*Sqrt[1 + (1 + 2*I)*Tan[t/2]^2] + (2 + I)*(Tan[t/2] + 2*Tan[t/2]^3 + 5*Tan[t/2]^5))))/((1 + Cos[t])*Sqrt[(5 - 4*Cos[t] + Cos[2*t])/(1 + Cos[t])^2]*(1 + Tan[t/2]^2)*Sqrt[2 + 4*Tan[t/2]^2 + 10*Tan[t/2]^4]) In[10]:= FullSimplify[ Limit[indef, t -> Pi, Direction -> 1] - (indef /. t -> 0)] Out[10]= (1/5)*Sqrt[2 + 4*I]*(-5*I*EllipticE[-(3/5) - (4*I)/5] + (2 + I)*Sqrt[5]*EllipticE[-(3/5) + (4*I)/5] - (12 - 4*I)*EllipticK[-(3/5) - (4*I)/5] + (6 - 2*I)*Sqrt[5]*EllipticK[8/5 - (4*I)/5] + 4*I*Sqrt[5]*EllipticPi[1/5 + (2*I)/5, -(3/5) + (4*I)/5] + (8 + 4*I)*EllipticPi[1 - 2*I, -(3/5) - (4*I)/5]) In[11]:= N[%] Out[11]= 6.722879723440325 + 1.0534455252564358*^-14*I Of course I readily agree that Out[10] is not as nice in appearance as your In[3] below. (Nobody who works much with Mathematica and elliptic integrals would be surprised by that.) Nonetheless, Out[10] is a correct answer. Best regards, David W. Cantrell > In[2]:= NIntegrate[Sqrt[5 - 4*Cos[t] + Cos[2*t]], {t, 0, Pi}] > Out[2]= 6.72288 > > I know the exact result: > > In[3]:= (1/5^(3/4))*(Sqrt[2]*(10*EllipticE[(1/10)*(5 - Sqrt[5])] - > 10*EllipticK[(1/10)*(5 - Sqrt[5])] + (5 + 3*Sqrt[5])* > EllipticPi[(1/10)*(5 - 3*Sqrt[5]), (1/10)*(5 - Sqrt[5])]))//N > Out[3]= 6.72288 > > but I would like to prove it. > > Thanks in advance to the samaritan experts... > > V.Astanoff