ODE's again (sorry)

["Sam Halliday" <plendily at hotmail dot com>]

hello all,

i have been having problems with the ODE's of gsl 1.1. i thought i had it 
mastered when i solved lane-emden (which is not disimilar to the 
thomas-fermi equation), but now i have moved onto solving 3 simple newtonian 
motion equations, which need to have solutions from the other in order to 
calculate the next value...

so i tried making 3 calls to the runge-kutta (4th order), which worked for 
me before, and if i solve for one of these 3 equations by itself, it seems 
to work, however if i change the step range in my solution to only solve for 
1 value (instead of a whole range), i get a slight error in the 2nd 
calculated values with respect to the solution over a range, here is some 
sample data
--solution by one call to my function (similar to the fixed-step example in 
the docs), solving for the whole range:

index	x		dx/dt
0       1.000000e+01    0.000000e+00
1       9.998821e+00    -2.946330e-01
2       9.994009e+00    -6.482472e-01 ...and on and on..

--solution by inserting results of a single step call as the initial values 
of next call:

index	x		dx/dt
0       1.000000e+01    0.000000e+00
1       9.998821e+00    -2.946330e-01
2       9.995188e+00    -5.893008e-01

as you can see, the problem lies with the second call, the dx/dt value is 
lower by making single calls, (which results in a higher x value) than by 
solving for a range.

maybe this is because i am not starting at t=0, can the gsl algorithms 
handle calls not started at t=0?

i realise this is quite a large request to expect somebody to help me solve 
it, but i am really at a loss and posting was my last resort. and i dont 
want to plague the list with my source code... unless you ask ;)

please help!
Sam
---
If you wish to be happy for one hour, get drunk.
If you wish to be happy for three days, get married.
If you wish to be happy for a month, kill your pig and eat it.
If you wish to be happy forever, learn to fish.
                -- Chinese Proverb

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