# Revision history [back]

### Has ticket #6480 been fixed?

The ticket is:

.subs_expr() method doesn't work for argument of D derivative operator


Here is an example (from Sage 6.2, which I'm currently using):

var('x,y,f,F,Fx,Fy')
y = function('y',x)
f = function('f',x,y)
d1 = diff(f,x,1)
d1.subs_expr(diff(y,x,1)==F)

F*D[1](f)(x, y(x)) + D[0](f)(x, y(x))


So far, all good. But when I try:

d1.subs_expr(diff(y,x,1)==F,diff(f,x,1)==Fx,diff(f,y,1)==Fy)


I receive the error:

TypeError: argument symb must be a symbol


### Has ticket #6480 been fixed?

The ticket is:

 .subs_expr() method doesn't work for argument of D derivative operator

operator

Here is an example (from Sage 6.2, which I'm currently using):

var('x,y,f,F,Fx,Fy')
y = function('y',x)
f = function('f',x,y)
d1 = diff(f,x,1)
d1.subs_expr(diff(y,x,1)==F)

F*D[1](f)(x, y(x)) + D[0](f)(x, y(x))


So far, all good. But when I try:

d1.subs_expr(diff(y,x,1)==F,diff(f,x,1)==Fx,diff(f,y,1)==Fy)


I receive the error:

TypeError: argument symb must be a symbol


Maybe my syntax is wrong, so either my question is moot, in which case what am I doing wrong? Or maybe my syntax is correct and the fault is still a ticket for subs_expr. Advice as always would be very welcome!