### Part I

So, yesterday I was wondering about operations on a set of sets (I’ll explain the Tweet content below, so you can just ignore it for now).

when we have a set of sets S, an element belongs to the:

— Vitor Enes (@vitorenesduarte) November 10, 2018

- union of S, if it belongs to at least 1 sets in S

- intersection of S, if it belongs to at least |S| sets in S

- _____ of S, if it belongs to at least N sets in S

what should I write in _____?

For example, consider that we have 3 sets:

- $A = \{a, b, c\}$
- $B = \{a, b, d\}$
- $C = \{a\}$

and we build a set $S = \{A, B, C\}$ with the previous sets.

The **union** of all sets has all elements:

$$\bigcup S = \{a, b, c, d\}$$

The **intersection** of all sets has the elements common to all sets:

$$\bigcap S = \{a\}$$

Now, we can generalize these definitions and have:

- an element belongs to $\bigcup S$ if it belongs to at least 1 set in $S$
- an element belongs to $\bigcap S$ if it belongs to at least $|S|$ sets in $S$
^{1}

Once we have this, we can define $\bigcup_n$:

- an element belongs to $\bigcup_n S$ if it belongs to at least $n$ sets in $S$

and $\bigcup$ and $\bigcap$ can be written in terms of $\bigcup_n$:

- $\bigcup S = \bigcup_1 S$
- $\bigcap S = \bigcup_{|S|} S$

So, my question was: *What’s the name of $\bigcup_n$*?

After much googling and no success, it came to mind that maybe this was not about operations on sets, but instead on multisets:

Maybe this is about multisets. If we build a multiset with S, then the elements of _____ S are those with multiplicity bigger or equal to N. https://t.co/qs58gi0Urs

— Vitor Enes (@vitorenesduarte) November 10, 2018

Unlike a set, a **multiset**, (and quoting Wikipedia) “*allows for multiple instances for each of its elements*”.

In a multiset we keep count of the number of instances of each element - this number is called **multiplicity**. I’ll use $e_m$ to represent an element $e$ with multiplicity $m$.

So, with sets $A$, $B$ and $C$, we can build a multiset $\{a_3, b_2, c_1, d_1\}$.

Using these concepts, we can redefine $\bigcup_n$:

- an element belongs to $\bigcup_n S$ if its multiplicity in the multiset is at least $n$

### Part II

Today, I was back on my quest. This concept should be something classical, with like 100 years. I was convinced that with this new insight (the multiset), I would finally find the name of $\bigcup_n$.

What was I not expecting, was to find it in a recent (2005) multi-party computation (MPC) paper. The paper is called **Privacy-Preserving Set Operations** and is authored by Lea Kissner (@LeaKissner) and Dawn Song (@dawnsongtweets).

The high-level idea of MPC is to have a set of players computing some function on some inputs. The inputs are to be kept private, and only the function output should be revealed.

So, if the inputs are the sets $A, B, C$ in $S$, and we’re computing their intersection, then only $\bigcap S = \{a\}$ should be known in the end.

In Section 6.2, I found the following^{2}:

We define the

Threshold Set-Unionproblem as follows: all players learn which elements appear in the combined private input of the players at least a threshold number $t$ times. For example, assume that $a$ appears in the combined private input of the players $15$ times. If $t = 10$, then all players learn $a$. However, if $t = 16$, then no player learns $a$.

With this, I declare my quest over, and I’ll be calling $\bigcup_n$ **threshold union**.

What do you think? Is there a better name? Has this concept been named elsewhere?