1 | initial version |

You should use instead:

```
sage: sum(x^i*ft.coefficient(x, i) for i in [0..5])
1/120*x^5 - 1/6*x^3 + x
```

*Explanation:* your original code involves **Sage's symbolic sum** and `i`

has to be a symbolic variable (you probably declared it as such via `var('i')`

). Now, the method `coefficient`

is expecting an integer, not a symbolic variable. For some reason, it returns always zero for a symbolic variable (IMHO an error message would be more appropriate here):

```
sage: i = var('i')
sage: ft.coefficient(x, i)
0
```

Hence the result that you get. On the contrary, the command

```
sum(x^i*ft.coefficient(x, i) for i in [0..5])
```

involves **Python's sum**, for which `i`

is an integer, so that `ft.coefficient`

behaves correctly. For more details, see the documentation returned by `sum?`

(in particular the warning in it).

Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.