1 | initial version |

The reason is that Sage considers that `Ds`

still has one variable, though it is a dummy one:

```
sage: Ds
t5040*sum((-1)^l/factorial(l), l, 0, 7)
sage: Ds.variables()
(l,)
```

You can solve this issue by simplifying the expression:

```
sage: Ds.simplify_full()
1854
```

2 | No.2 Revision |

The reason is that Sage considers that `Ds`

still has one variable, though it is a dummy one:

```
sage: Ds
t5040*sum((-1)^l/factorial(l), l, 0, 7)
sage: Ds.variables()
(l,)
```

You can solve this issue by simplifying the expression:

```
sage: Ds.simplify_full()
1854
```

By the way, please note that the lines:

```
D=function('D')(n,l)
Dn=function('Dn')(n,l)
```

are useless, since `D`

and `Dn`

are overwritten just after.

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