用Python解Lotka-Volterra方程

Question

我正在尝试将一段MATLAB代码转换为Python3代码。

代码与Lotka-Volterra方程有关

这是MATLAB的原始代码

t0=0; tf=6*pi; eps=0.1; delta=0;
r=[1/2, 1, 2];
options = odeset(’RelTol’,1e-6,’AbsTol’,1.e-9);

%phase space plot
xpos=[2.5 2.5 2.5]; ypos=[3.5 3.5 3.5]
for i=1:length(r);
    for eps=0.1:0.1:1.0;
       [t,UV]=ode45(@(t,UV) lv_eq(t,UV,r(i)),[t0,tf],[1+eps 1+delta],options);
        U=UV(:,1); V=UV(:,2);
    figure(2);subplot(1,3,i); plot(U,V); hold on;
    end
    axis equal; axis([0 4 0 4]);
    text(xpos(i),ypos(i),[’r=’,num2str(r(i))]);
    if i==1; ylabel(’predator’); end;
    xlabel(’prey’);
end

function dUV=lv_eq(t,UV,r)
dUV=zeros(2,1);
dUV(1) = r*(UV(1)-UV(1)*UV(2));
dUV(2) = (1/r)*(UV(1)*UV(2)-UV(2));

以下是我的Python3代码：

import numpy as np
import matplotlib.pyplot as plt
from scipy import integrate
from functools import partial

def lotka_volterra(t, X, r):
    return np.array([r*(X[0]-X[0]*X[1]), (1/r)*(X[0]*X[1]-X[1])])


t0=0
tf=6*np.pi
prey_0 = 0.1
predator_0 = 0
rs = [0.5, 1, 2]


fig, axes = plt.subplots(ncols=3, sharey=True)
for i in range(len(rs)):
    for prey_0 in np.arange(0.1, 1.0, 0.1):
        f = partial(lotka_volterra, r=rs[i])
        sol = integrate.solve_ivp(f, t_span=[t0, tf], y0=[1+prey_0, 1+predator_0], rtol=1e-6, atol=1.e-9)
        U, V = sol.y
        axes[i].plot(U, V)

    axes[i].axis([0, 4, 0, 4])
    axes[i].set_title('r='+str(rs[i]))
    axes[i].set_xlabel('prey')
    if i==0:
        axes[i].set_ylabel('predator')

原始代码的输出为：

我的代码的输出是：

我不确定如何解决此问题。