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How do I perform a change of variables for a pde

asked 2011-11-03 17:02:07 +0200

rtrwalker gravatar image

How can I transform, step by step, the partial differential equation (pde) u.diff(z,z)k/gw == mvu.diff(t) into u.diff(Z,Z)=u.diff(Tv) where Z=z/H, Tv = cvt/H^2 and cv = k/(gwmv)? As you can see from my attempt below I can isolate cv; after that I have no idea how to do the change of variables. I can do this example by hand: let Z=z/H --> du/dz = du/dZ * dZ/dz --> du/dz = du/dZ *(1/H) ...etc. But Other examples may not be so easy (and I don't want to do it by hand!).

u = function('u',z,t)
eq1 = u.diff(z,z)*k/gw == mv * u.diff(t)
eq2 = eq1.divide_both_sides(mycoeff)
mycoeff = eq2.right_hand_side().coeff(u.diff(t),1)
eq3 = cv == 1/mycoeff
eq4 = solve(eq3,eq3.right_hand_side().default_variable(), solution_dict=true)
eq5 = eq2.subs(eq4[0])

which gives me the following output:

k*D[0, 0](u)(z, t)/gw == mv*D[1](u)(z, t)
D[0, 0](u)(z, t) == gw*mv*D[1](u)(z, t)/k
cv == k/(gw*mv)
[{gw: k/(cv*mv)}]
D[0, 0](u)(z, t) == D[1](u)(z, t)/cv
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answered 2011-11-14 20:59:45 +0200

rtrwalker gravatar image

updated 2011-11-14 21:02:39 +0200

With some insights from the following posts:

substitute expression instead of formal function symbol

How do I get an ordered list of a symbolic functions arguments?

Call a function with argument list in python (non-sage link)

I've come up with a function, transform_func_x(expr,f,rels), to change the dependent variables of an expression. Below is the function itself and below that how I've used it (all copy and pasted from a sage notebook). Any suggested improvements are welcome.

def transform_func_x(expr,f,rels):
    Transform the dependent variables of a symbolic function


    - ``expr`` - symbolic function.  Usually a relational expression
      with differential operators such as a partial differential equation.

    - ``f`` -  expression defining a function (e.g. previously defined
      as f = function ('u', z, t))

    - ``rels`` - list of relational expressions giving variable
      transformations.  For example to change y(z, t) to y(Z, T_v), rels
      could be [Z == z/H, T_v = c_v * t / H^2].  To change y(z, t) to y(Z, t),
      rels could be [Z == z / H]

    .. note::

       1. Transformations will not be performed for any relationship where the
       LHS is already one of the variables in f. For, example if
       f = u(Z, t) and rels = [Z == z / H, T_v == c_v * t / H^2] then the
       "Z == z / H" transformation will be ignored.
       2. After applying this function to expr, you will probably want to 
       update f by applying transform_func_x to f itself 
       i.e transform_func_x(f,f,rels)
    y = f.operator()
    x = f.operands()
    nx = f.number_of_operands()

    new_arg_list = x[:]
    """used to transform variables within f to functions
       of those variables. e.g. u(z, t) --> u(z / H, t)"""
    subs_dict1 = {}
    """used to change name of transformed variables within f.
       e.g. u(z / H, t) --> u(Z, t)"""
    subs_dict2 = {}
    """used to transform variables outside of f.
       e.g. z *u(Z, t)--> z * H * u(Z, t)"""
    for v in rels:
        if v.left() not in x:
            """i.e. exclude transformations where LHS of
               rels is already a variable of f"""
            for i in range(nx):
                # find which variable the transformation applies to
                if x[i] in v.right().variables():
            new_arg_list[xi] = v.right()
            subs_dict2[x[xi]]=solve(v,x[xi],solution_dict=true)[_sage_const_0 ][x[xi]]

    # make dummy function for transforming variables of f
    dummy_arg_list = [var("x{0:d}".format(i)) for i in range(nx)]
    dummy_y(*dummy_arg_list) = function(str(y),*new_arg_list))

    return (expr.substitute_function(y,dummy_y).subs(subs_dict1).

var('z,t,H,T_v,Z,c_v,k_v,m_v, g_w')
pde = k_v*f.diff(z,z)/g_w == m_v*f.diff(t)

print('Variable transformations')

print('PDE with u(z,t):')
print('Term to isolate:')
myterm = f.diff(f.operands()[0],2)
print (myterm)
print('Coefficient of term on LHS:')
print(mycoeff ...
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BTW, many thanks for coming back and posting your solution! Often people forget about a question and leave it hanging, but this helps everybody.

DSM gravatar imageDSM ( 2011-11-16 10:03:55 +0200 )edit

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Asked: 2011-11-03 17:02:07 +0200

Seen: 2,111 times

Last updated: Nov 14 '11