# Revision history [back]

This is tricky, but it can be done. The idea is to replace (in the expression tree) derivatives of $f$ with respect to $t$ by the right-hand side of the equation, substituted into itself as many times as there are derivatives with respect to $t$ minus one, and finally replacing $f$ by the derivative of $f$ with respect to the remaining variables (if any).

My attempt to implement this (based on SubstituteFunction) is below, with an example (your example) in the docstring.

from sage.symbolic.expression_conversions import ExpressionTreeWalker
from sage.symbolic.operators import FDerivativeOperator

class SubstituteEvolutionaryPDE(ExpressionTreeWalker):
def __init__(self, ex, pde):
"""
A class that walks the tree and replaces derivatives of f with
respect to t by the right-hand side of an evolutionary PDE.
EXAMPLES::
sage: var('x,t'); f = function('f'); g = function('g')
sage: pde = diff(f(t,x),t) == 1/2*diff(f(t,x),x,x)
sage: s = SubstituteEvolutionaryPDE(0, pde)
sage: h = g(diff(f(t,x),t), diff(f(t,x),t,x))
sage: s(h)
g(1/2*diff(f(t, x), x, x), 1/2*diff(f(t, x), x, x, x))

ASSUMPTION::

pde is of the form diff(f(t,...),t) == ... and the first
argument of f is always t.
"""
self.f = integrate(pde.lhs(), t).operator()
self.rhs = pde.rhs()
self.ex = ex

def derivative(self, ex, operator):
if operator.function() == self.f and 0 in operator.parameter_set(): # f is differentiated with respect to t
t_derivatives = [p for p in operator.parameter_set() if p == 0] # (assumes t is the first argument)
result = self.rhs
for _ in range(len(t_derivatives)-1):
result = result.substitute_function(self.f, self.rhs)
if len(t_derivatives) < len(operator.parameter_set()): # derivatives w.r.t variables other than t
other_derivatives = [p for p in operator.parameter_set() if not p == 0] # (assumes t is the first argument)
new_operator = FDerivativeOperator(operator.function(), other_derivatives)
result = result.substitute_function(self.f, new_operator(*[self(_) for _ in ex.operands()]))
return result
else:
return operator(*[self(_) for _ in ex.operands()])


This is tricky, but it can be done. The idea is to replace (in the expression tree) derivatives of $f$ with respect to $t$ by the right-hand side of the equation, substituted into itself as many times as there are derivatives with respect to $t$ minus one, and finally replacing $f$ by the derivative of $f$ with respect to the remaining variables (if any).

My attempt to implement this (based on SubstituteFunction) is below, with an example (your example) in the docstring.

docstring.
from sage.symbolic.expression_conversions import ExpressionTreeWalker
from sage.symbolic.operators import FDerivativeOperator

FDerivativeOperator class SubstituteEvolutionaryPDE(ExpressionTreeWalker):
def __init__(self, ex, pde):
pde, t):
"""
A class that walks the tree and replaces derivatives of f with
respect to t by the right-hand side of an evolutionary PDE.
EXAMPLES::
sage: var('x,t'); f = function('f'); g = function('g')
sage: pde = diff(f(t,x),t) == 1/2*diff(f(t,x),x,x)
sage: s = SubstituteEvolutionaryPDE(0, pde)
pde, t)
sage: h = g(diff(f(t,x),t), diff(f(t,x),t,x))
sage: s(h)
g(1/2*diff(f(t, x), x, x), 1/2*diff(f(t, x), x, x, x))

ASSUMPTION::

pde is of the form diff(f(t,...),t) == ... and the first
argument of f is always t.
"""
self.f = integrate(pde.lhs(), t).operator()
self.rhs = pde.rhs()
self.ex = ex

def derivative(self, ex, operator):
if operator.function() == self.f and 0 in operator.parameter_set(): # f is differentiated with respect to t
t_derivatives = [p for p in operator.parameter_set() if p == 0] # (assumes t is the first argument)
result = self.rhs
for _ in range(len(t_derivatives)-1):
result = result.substitute_function(self.f, self.rhs)
if len(t_derivatives) < len(operator.parameter_set()): # derivatives w.r.t variables other than t
other_derivatives = [p for p in operator.parameter_set() if not p == 0] # (assumes t is the first argument)
new_operator = FDerivativeOperator(operator.function(), other_derivatives)
result = result.substitute_function(self.f, new_operator(*[self(_) for _ in ex.operands()]))
return result
else:
return operator(*[self(_) for _ in ex.operands()])


 3 No.3 Revision updated 2018-11-01 17:39:25 +0200 This is tricky, but it can be done. The idea is to replace (in the expression tree) derivatives of $f$ with respect to $t$ by the right-hand side of the equation, substituted into itself as many times as there are derivatives with respect to $t$ minus one, and finally replacing $f$ by the derivative of $f$ with respect to the remaining variables (if any). My attempt to implement this (based on SubstituteFunction) is below, with an example (your example) in the docstring. docstring. from sage.symbolic.expression_conversions import ExpressionTreeWalker from sage.symbolic.operators import FDerivativeOperator FDerivativeOperator class SubstituteEvolutionaryPDE(ExpressionTreeWalker): def __init__(self, ex, pde, t): """ A class that walks the tree and replaces derivatives of f with respect to t by the right-hand side of an evolutionary PDE. EXAMPLES:: sage: var('x,t'); f = function('f'); g = function('g') sage: pde = diff(f(t,x),t) == 1/2*diff(f(t,x),x,x) sage: s = SubstituteEvolutionaryPDE(0, pde, t) sage: h = g(diff(f(t,x),t), diff(f(t,x),t,x)) sage: s(h) g(1/2*diff(f(t, x), x, x), 1/2*diff(f(t, x), x, x, x)) ASSUMPTION:: pde is of the form diff(f(t,...),t) == ... and the first argument of f is always t. """ self.f = integrate(pde.lhs(), t).operator() self.rhs = pde.rhs() self.ex = ex def derivative(self, ex, operator): if operator.function() == self.f and 0 in operator.parameter_set(): # f is differentiated with respect to t t_derivatives = [p for p in operator.parameter_set() if p == 0] # (assumes t is the first argument) result = self.rhs for _ in range(len(t_derivatives)-1): result = result.substitute_function(self.f, self.rhs) if len(t_derivatives) < len(operator.parameter_set()): # derivatives w.r.t variables other than t other_derivatives = [p for p in operator.parameter_set() if not p == 0] # (assumes t is the first argument) new_operator = FDerivativeOperator(operator.function(), other_derivatives) result = result.substitute_function(self.f, new_operator(*[self(_) for _ in ex.operands()])) return result else: return operator(*[self(_) for _ in ex.operands()]) 4 No.4 Revision updated 2018-11-01 18:18:05 +0200 This is tricky, but it can be done. The idea is to replace (in the expression tree) derivatives of $f$ (with at least one derivative with respect to $t$ $t$) by the right-hand side of the equation, substituted into itself as many times as there are derivatives with respect to $t$ minus one, and finally replacing $f$ by the derivative of $f$ with respect to the remaining variables (if any). My attempt to implement this (based on SubstituteFunction) is below, with an example (your example) in the docstring. from sage.symbolic.expression_conversions import ExpressionTreeWalker from sage.symbolic.operators import FDerivativeOperator class SubstituteEvolutionaryPDE(ExpressionTreeWalker): def __init__(self, ex, pde, t): """ A class that walks the tree and replaces derivatives of f with respect to t by the right-hand side of an evolutionary PDE. EXAMPLES:: sage: var('x,t'); f = function('f'); g = function('g') sage: pde = diff(f(t,x),t) == 1/2*diff(f(t,x),x,x) sage: s = SubstituteEvolutionaryPDE(0, pde, t) sage: h = g(diff(f(t,x),t), diff(f(t,x),t,x)) sage: s(h) g(1/2*diff(f(t, x), x, x), 1/2*diff(f(t, x), x, x, x)) ASSUMPTION:: pde is of the form diff(f(t,...),t) == ... and the first argument of f is always t. """ self.f = integrate(pde.lhs(), t).operator() self.rhs = pde.rhs() self.ex = ex def derivative(self, ex, operator): if operator.function() == self.f and 0 in operator.parameter_set(): # f is differentiated with respect to t t_derivatives = [p for p in operator.parameter_set() if p == 0] # (assumes t is the first argument) result = self.rhs for _ in range(len(t_derivatives)-1): result = result.substitute_function(self.f, self.rhs) if len(t_derivatives) < len(operator.parameter_set()): # derivatives w.r.t variables other than t other_derivatives = [p for p in operator.parameter_set() if not p == 0] # (assumes t is the first argument) new_operator = FDerivativeOperator(operator.function(), other_derivatives) result = result.substitute_function(self.f, new_operator(*[self(_) for _ in ex.operands()])) return result else: return operator(*[self(_) for _ in ex.operands()]) 5 No.5 Revision updated 2020-02-14 23:18:57 +0200 This is tricky, but it can be done. The idea is to replace (in the expression tree) derivatives of $f$ (with at least one derivative with respect to $t$) by the right-hand side of the equation, substituted into itself as many times as there are derivatives with respect to $t$ minus one, and finally replacing $f$ by the derivative of $f$ with respect to the remaining variables (if any). My attempt to implement this (based on SubstituteFunction) is below, with an example (your example) in the docstring. from sage.symbolic.expression_conversions import ExpressionTreeWalker from sage.symbolic.operators import FDerivativeOperator class SubstituteEvolutionaryPDE(ExpressionTreeWalker): def __init__(self, ex, pde, t): """ A class that walks the tree and replaces derivatives of f with respect to t by the right-hand side of an evolutionary PDE. EXAMPLES:: sage: var('x,t'); f = function('f'); g = function('g') sage: pde = diff(f(t,x),t) == 1/2*diff(f(t,x),x,x) sage: s = SubstituteEvolutionaryPDE(0, pde, SubstituteEvolutionaryPDE(pde, t) sage: h = g(diff(f(t,x),t), diff(f(t,x),t,x)) sage: s(h) g(1/2*diff(f(t, x), x, x), 1/2*diff(f(t, x), x, x, x)) ASSUMPTION:: pde is of the form diff(f(t,...),t) == ... and the first argument of f is always t. """ f_expr = integrate(pde.lhs(), t) self.f = integrate(pde.lhs(), t).operator() f_expr.operator() self.args = f_expr.operands() self.rhs = pde.rhs() self.ex = ex def derivative(self, ex, operator): if operator.function() == self.f and 0 in operator.parameter_set(): # f is differentiated with respect to t t_derivatives = [p for p in operator.parameter_set() if p == 0] # (assumes t is the first argument) result = self.rhs for _ in range(len(t_derivatives)-1): result = result.substitute_function(self.f, self.rhs) self.rhs.function(*self.args)) if len(t_derivatives) < len(operator.parameter_set()): # derivatives w.r.t variables other than t other_derivatives = [p for p in operator.parameter_set() if not p == 0] # (assumes t is the first argument) new_operator = FDerivativeOperator(operator.function(), other_derivatives) result = result.substitute_function(self.f, new_operator(*[self(_) for _ in ex.operands()])) ex.operands()]).function(*self.args)) return result else: return operator(*[self(_) for _ in ex.operands()]) Edit: updated to use substitute_function correctly (the second argument should also be a function). 


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