### function paramenters as sum limits

I am completely new to sage, so I am afraid that this should be a very standard well known issue. Sorry for that.

I need to define a function one of whose parameters is a limit of a sum.
I tried:

```
d,n,i = var('d,n,i')
def N(d,n):
if n==1:
return 1
else:
return sum(N(d,i),i,1,n-1)
```

But sage complains with a RuntimeError. Why is that? I suppose that for some reason the parameter `n`

from the function is not assigned to the variable limit `n-1`

in the sum. I this correct? How can I fix that?