CODEWITHSUNDEEP
pythonscipyode
John
John

Reputation: 497

scipy solve_ivp with negative first_step

I want to solve a system of ODEs with scipy's solve_ivp and I need a negative first_step in order to evolve the solution, but 

Nsol = solve_ivp(derivs, (N , Nend), ydoub, method='RK45', t_eval=None, dense_output=False, events=None, vectorized=False, first_step=-1e-8)

returns

ValueError: `first_step` must be positive.

Any ideas how to resolve this or find a way around it?

EDIT: here is the code which yields this:

import numpy
from scipy.integrate import solve_ivp

w = numpy.array ( [  0.00000000e+00,   1.00000000e+00,   3.17214587e-01,
        -3.41988549e-01,  -1.50137165e-05,  -2.48117074e-02,
         1.17624224e-03,  -1.27149037e-04] )

def derivs2 (t, w):
    dydN = numpy.zeros(2 , dtype=float , order='C')

    dydN[0] = 0.0

    dydN[1] = y[1] * y[2]
    dydN[2] = y[2] * ( y[3] + 2.0 * y[2] )
    dydN[3] = 2.0 * y[4] - 5.0 * y[2] * y[3] - 12.0 * y[2] * y[2]

    for i in range (4 , NEQS-1):
        dydN[i] = ( 0.5 * (i-3) * y[3] + (i-4) * y[2] ) * y[i] + y[i+1]

    dydN[NEQS-1] = ( 0.5 * (NEQS-4) * y[3] + (NEQS-5) * y[2] ) * y[NEQS-1]

    return dydN

Nsol = solve_ivp(derivs, (1000.0 , 0.0), w, method='RK45', t_eval=None, dense_output=False, events=None, vectorized=False , first_step=-1e-6)

Upvotes: -1

Views: 3579

Answers (1)

Matthew Flamm
Matthew Flamm

Reputation: 595

Your code has many problems (derivs vs. derivs2 and w vs y, etc.) and will not run.

first_step is actually the magnitude of the first step, this is not described clearly, or at all, in the documentation. Change first_step=1e-6 and this should work.

from scipy.integrate import solve_ivp 

def fun(t, y): 
    return y 

try: 
    sol = solve_ivp(fun, (1000, 0), [1], first_step=-1e-6) 
except ValueError: 
    print(f"fails backwards") 

sol = solve_ivp(fun, (1000, 0), [0], first_step=1e-6) 
print(sol.t) 
print("First step = {}".format(sol.t[1]-sol.t[0]))

result:

fails backwards
[1000.          999.999999    999.999989    999.999889    999.998889
  999.988889    999.888889    998.888889    988.88888903  888.88888928
    0.        ]
First step = -9.999999974752427e-07

Upvotes: 2

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