Avoiding imaginary numbers in DSolve
- To: mathgroup at smc.vnet.net
- Subject: [mg37977] Avoiding imaginary numbers in DSolve
- From: "Dave Snead" <dsnead6 at charter.net>
- Date: Sat, 23 Nov 2002 19:14:54 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
I'm using DSolve to solve a differential equation, the result of which
should be real but instead I get combinations
of quantities involving the imaginary I (which I assume reduces to a real
quantity). How do I get results that look
real and avoid I? Thanks in advance.
(FullSimplify[#1, M > 0 && r > 0] & )[
DSolve[(-Derivative[2][t][r])*r^2 +
M*Derivative[1][t][r]^3 == 0, t[r], r]]
{{t[r] -> Sqrt[r*(M - r*C[1])]/(Sqrt[2]*C[1]) + C[2] -
(I*M*Log[2*((-I)*Sqrt[2]*Sqrt[r*C[1]] +
Sqrt[2*M - 2*r*C[1]])])/(Sqrt[2]*C[1]^(3/2))},
{t[r] -> -(Sqrt[r*(M - r*C[1])]/(Sqrt[2]*C[1])) + C[2] +
(I*M*Log[2*((-I)*Sqrt[2]*Sqrt[r*C[1]] +
Sqrt[2*M - 2*r*C[1]])])/(Sqrt[2]*C[1]^(3/2))}}