Re: [Maxima-discuss] rkf45 fails in 2d?

[Helmut J. <jar...@ig...>]

On 04/05/2014 11:16:22 PM, Leo Butler wrote:
> Leo Butler <l_b...@us...> writes:
> 
> > "Edwin Woollett" <woo...@ch...> writes:
> >
> >> I have tried out rkf45 with success on several problems
> >> but am getting a  weird error return  (initial should be
> >>   a list of numbers)   with a driven
> >> simple harmonic oscillator case. The built-in rk()
> >> function returns a solution without complaint.
> >> --------------------------------------
> >> (%i1) fpprintprec:7$
> >> (%i2) load(rkf45);
> >> (%o2)
> >>  
> "C:/PROGRA~1/MAXIMA~3.2/share/maxima/5.31.2/share/contrib/rkf45/rkf45.mac"
> >> (%i3) dx_dt : vx$
> >> (%i4) dvx_dt : -4*%pi^2*x + 100*sin(4*%pi*t + %pi/4)/(0.5 + t^2)$
> >
> > After looking at rkf45.mac, the error is being thrown because  
> dvx_dt is
> > not evaluating to a float. If you add the numer option to %i4, then  
> %pi
> > will be coerced to a float and the integration will succeed.
> >
> > The error message needs to be improved because I think the  
> intention is
> > to check to see if the odes will evaluate to floats at the initial
> > conditions.
> 
> Ted, when I replace line 112 of rkf45.mac by the following 3 lines
> 
> if member(false,map(numberp,initial)) then  error("Error: initial  
> should be a list of numbers, but found",initial,"instead."),
> block([initial_values:apply(f_rhs,cons(xi,yi))],
>   if member(false,map(numberp,initial_values)) then error("Error:  
> odes should evaluate to a list of numbers at initial, but  
> found",initial_values,"instead.")),
> 
> I get the error message
> 
> Error: odes should evaluate to a list of numbers at initial, but found
>  [0,200.0*sin(0.25*%pi)-39.47841760435743] instead.
> #0:  
> rkf45(odes=[vx,100*sin(4*%pi*t+%pi/4)/(t^2+0.5)-4*%pi^2*x],funcs=[x,vx],initial=[1,0],interval=[t,0,1],options=[])(rkf45.mac  
> line 114)
> 
> Can you tell me if you see the same thing when you make the  
> alteration?
> 
> Leo
> 

Hi,
having applied Leo's patch I get the strange error messages which  
probably needs
a similar patch:

(%i9) rkf45(odes = [vx,100*sin(%pi/4+4*%pi*t)/(0.5+t^2)-4*%pi^2*x],
             funcs = [x,vx],initial = [1,0],interval = [t,0,1],options  
= [])
Error: interval should be a list, but found interval = [t, 0, 1]  
instead.
#0: rkf45(odes=odes =  
[vx,100*sin(4*%pi*t+%pi/4)/(t^2+0.5)-4*%pi^2*x],funcs=funcs =  
[x,vx],initial=initial = [1,0],interval=interval =  
[t,0,1],options=[options = []])(rkf45.mac line 94)

Helmut