1 | initial version |

You should use the method `substitute_function`

instead of `subs`

and use a symbol different from `p`

to denote `p(r)`

(otherwise there is a confusion between the function `p`

and its value at `r`

). Besides the semicolons at the end of each line are not necessary in Python and the line `v = var('v')`

is useless, since the Python variable `v`

is redeclared in the next line. So basically, your code should be:

```
sage: r = var('r')
sage: P = function('p')(r)
sage: v = P + P.derivative(r)
sage: p1(r) = desolve(P.derivative(r)==0, P, ivar=r); p1(r)
_C
sage: v.substitute_function(p, p1)
_C
```

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