How to express ODE?
- To: mathgroup at smc.vnet.net
- Subject: [mg52817] How to express ODE?
- From: Adam Getchell <agetchell at physics.ucdavis.edu>
- Date: Mon, 13 Dec 2004 04:24:15 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Still (unfortunately) not getting another ODE to work, even using Dr.
Bob's example. I want to express this equation:
In[143]:=
m = 10^16;
mp = 1.2211*10^19;
G = mp^(-2);
V[\[Phi]_] := (1/2)*m^2*\[Phi][t]
\[Rho][\[Phi]_] :=
Derivative[1][\[Phi]][t]^2/2 +
V[\[Phi]]
H := Sqrt[((8*Pi*G)/3)*
\[Rho][\[Phi]]]
In[149]:=
Inflaton :=
Derivative[2][\[Phi]][t] +
3*H*Derivative[1][\[Phi]][
t] + D[V[\[Phi]], \[Phi]]
That is, phi double-dot of t + 3 H phi dot of t + V' of phi, where phi
is a function of t; V is a function of phi; H is a function of rho; rho
is a function of phi dot and V.
However, solving Inflaton for phi as a function of t:
In[163]:=
f = \[Phi] /. First[NDSolve[
{Inflaton == 0,
Derivative[1][\[Phi]][
t] == 0}, \[Phi],
{t, 0, 100*m}]]
Generates NDSolve::overdet and ReplaceAll::reps.
I *think* part of my problem is that dV/d(phi) is not getting properly
evaluated (it returns 0 when I look at it explicitly).
I thought that Derivative[1][\[Phi]][x] would give the above, but not
the way I'm doing it.
--Adam