ODE interpolation

[soroush]

I want write ROOT or c++ code of this python script:

def ode_func(x, t,a):
    return -(3*pow(1+t,-1)*(a-1))*x*((1-x)/(1-2*x+a*x))

sol=lambda a,inits: odeint(ode_func,inits,t,args=(a,)) 

def model(a,inits):
    y = np.ravel(sol(a,inits))
    return interp1d(t, y, kind='cubic')

def integrand(z, omm,a):
    inits=1-omm
    f2= model(a,inits)
    Ez = 1.0/np.sqrt((omm*(1.0+z)**3 + f2(z)))
    return Ez
def integ(z, omm,a):
    integral = integrate.quad(comoving_integrand, 0, z, args=(omm,a))[0]
    return 300000*(1+z)*integral 

first solve ODE , interpolated over interval and then use as integrand function “comoving_integrand” and finally integrate it.

Any suggestions would be greatly appreciated

[jalopezg]

Hi soroush,

I this this can be done with RDataFrame. I will involve @eguiraud, maybe he knows the answer.

Cheers,
J.

[soroush]

Hi jalpezg
Thank s for the reply.Is it possible I mention @eguiraud for answering or I should change category to ROOTfit?

[jalopezg]

You do not have to perform any further action :-). I am also inviting @StephanH.

Cheers.

[soroush]

Thanks a lot.

[eguiraud]

Hi,
I’m afraid this is really a statistics/scientific computation problem. Maybe @moneta can help.

Cheers,
Enrico

[moneta]

Hi,
ROOT is not having any specific code for solving ODE. But there is code to integrate functions, so based on that you could maybe implement the code above in C++.
Otherwise, you can also use th GSL C library, (used internally in ROOT in the MathMore) library, that provides functions to solve ODE systems (see https://www.gnu.org/software/gsl/doc/html/ode-initval.html )

Lorenzo