# Runge Kutta 3 Method in Python (RK3) for matrices

I'm struggling to try and put my idea of what I have for this problem into Python, I'm stuck on trying to put the bvector(x) function to give me my required output.   • Your pictures 2 and 3 are the same picture. – Arthur Nov 1 '19 at 8:29
• What have you tried so far? What exactly is your problem? – G. Gare Nov 1 '19 at 8:47
• As an example, you could have bvector = lambda x: np.array([x+1,x**2]); as forcing term / right side / inhomogeneity. – Lutz Lehmann Nov 1 '19 at 10:55
• You might edit that fact into the question text above, preferably at the very start where it will be most likely to be noticed by anyone who sees this question. – David K Nov 1 '19 at 13:52

### bvector

This encodes the inhomogeneity in the linear system of differential equations $$y'(x)=A\,y(x)+b(x)$$ Thus $$b(x)$$ is a vector of the same dimension as $$y$$. As the vector addition in matlab and python has a mode of adding a scalar, adding it to all components, one can simplify the current case of a zero vector to just returning the scalar $$0$$.

bvector = lambda x: 0


If this vector is non-trivial, you would have to return a proper vector, for instance using numpy.array as vector type. In dimension two this could look like

bvector = lambda x: np.array([x+1,x**2]);


### RK3 step

If the current state is xn,yn then the step of size h is implemented as just repeating the (corrected) formulas,

rk3step(xn,yn,h):
y1 = yn+h*(A.dot(yn)+bvector(xn));
y2 = 0.75*yn+0.25*y1+0.25*h*(A.dot(y1)+bvector(xn+h));
return (yn+2*y2+2*h*(A.dot(y2)+bvector(xn+0.5*h)))/3;


### A possible loop

This you now can embed into a list construction loop

def rk3(A, bvector, y0, interval, N):
def rk3step: ...
x = np.linspace(*interval,N+1);
y = [y0]
for n in range(N):
y.append(rk3step(x[n],y[n],x[n+1]-x[n]));
return x,np.asarray(y).T