ISO style advice for nested sum with unique indices

Stream: new members

[Becker A.]

I want to write something like this:

$$\sum_{i \neq j \neq k} x_i \cdot x_j \cdot x_k$$

I've been using something like this:

∑ i1, ∑ i2 ∈ univ.erase i1, ∑ i3 ∈ (univ.erase i1).erase i2, f i1 * f i2 * f i3

is there a "cleaner" way of expressing this?

[Yaël Dillies]

You could sum over sets of size three: ∑ s ∈ .powersetCard 3 .univ, ∏ i ∈ s, x i

[Yaël Dillies]

This assumes x is valued in a commutative monoid