As a simple example, I have the following variables and functions:
r = var('r')
th = var('th', latex_name = '\\theta')
g = function('g')(r, th)
f = g + diff(g,r)
From now on I want to decompose g into g0and g2 as:
g0 = function('g0', latex_name = 'g_0')(r)
g2 = function('g2', latex_name = 'g_2')(r)
g = g0 + cos(th)*g2
If I print f I still get:
$ f = g(r, \theta) + frac{dg(r,\theta)}{dr} $
So instead, I apply a substitution on g:
f = f.subs(g==g0+cos(th)*g2)
This changes g, but not diff(g,r):
$f = g_0(r) + g_2(r)*\cos(\theta) + frac{dg(r,\theta)}{dr}$
In order to change also diff(g,r) I have to substitute explicitly the derivative:
f = f.subs(diff(g,r) == diff(g0+cos(th)*g2, r))
My question is the following:
Is there a way I can redefine a function without having to redefine also every single derivative? This way, I would avoid having to write all these substitutions:
diff(g,r) == diff(g0+g2*cos(th), r),
diff(g,r,r) == diff(g0+g2*cos(th), r, r),
diff(g,th) == diff(g0+g2*cos(th), th),
diff(g,th,th) == diff(g0+g2*cos(th), th, th),
diff(g,th,r) == diff(g0+g2*cos(th), th, r),
diff(g,r,th) == diff(g0+g2*cos(th), r, th)
