(extended comment)

probably you want to use `min_symbolic`

: the top-level min function that you are calling refers to Python's built-in `min`

(which is ok for numeric values).

in this case i get:

```
sage: var('i, k, la, lb')
(i, k, la, lb)
sage: sum(i, i, 0, min_symbolic(k-1, la+lb))
1/2*min(k - 1, la + lb)^2 + 1/2*min(k - 1, la + lb)
```

(of course you could have just substituted in $n(n+1)/2$ for $n=\text{min_symbolic}(k-1, la+lb)$).

PS: it's in the reference manual as Symbolic Maximum and Minimum, and there is some work on updating that section in ticket #11258.

probably you want to use

`min_symbolic`

: the top-level`min`

function that you are calling refers to Python's built-in min (which is ok for numeric values).Want to add that as an answer?