## Module A Supplement

Here are some general comments of mine on the exercises you handed in on
Friday. Review how I discuss these exercises and try to take advantage
of it in your self-evaluation.

##

###

### Unit A3 Exercise 2 (Drug Trafficking)

In this exercise, you were asked to represent a hypothetical drug trafficking scenario as a mathematical structure **as a combination of sets, relations, and functions**. Thus, you should be

A very simple hypothetical scenario might be as follows: Drug producers in some country sell drug to traffickers. The traffickers around the world sell drug to dealers in certain target countries. The dealers sell drug to users.

- Note: It was very good that you came up with all sorts of realistic
and/or revealing scenarios. More complex scenario will end up with
more complex representations. Here, I use a simple one to illustrate
how you can represent it in a way we discussed in class.

In this scenario, the key players include producers, traffickers, dealers, and users. They are all people. So, we can consider a set of people. The essential connection between these people is the act of selling. So, we can consider a relation "sells", i.e., "___ sells (drug) to ___".

Then, we have a mathematical structure, say, **DrugTrafficking**, consisting of the following components:

- Set of people: (for example) producer
_{1}, producer_{2}, trafficker_{1}, trafficker_{2}, ..., dealer_{i}, ... , user_{n}
- Relation "sells (drug) to":
- producer
_{1} sells (drug) to trafficker_{1}
- producer
_{1} sells (drug) to trafficker_{2}
- producer
_{2} sells (drug) to trafficker_{1}
- ...

- dealer
_{i}, sells (drug) to user_{n}

A relation connects two objects, in this case two people. As shown above,
if one individual is involved in two different people, you will need to show
both instances. You will also need to analyze whether this relation
is a function (you must be able to analyze this by observing that producer_{1} is involved with two different people).

Structure **DrugTrafficking** is different from several structures discussed in class. For example, it is different from **String**, **PrimitiveCounting**, and **Max**
because the structure patterns are different (the latter structures involve
a function, not a relation). It is different from **BarbieDoll** because the latter involves multiple relations and also a function. However, it is similar to a few structures, e.g., **ObjectsInRoom**, **NorthPole**, **Professional**, **Map**.
All of these involve a set and a relation (which cannot be in general
a function). In fact, this kind of structure is called **Graph**, as we discussed in class. Although I did not clarify in class, **DrugTrafficking** is in general not a **Tree**
(check the properties shown on slide) because it is quite possible that there
are disconnected groups of people (e.g., a drug trafficking links from South
America to the US and another entirely separate one locally taking place
in a remote community near the Arctic)._{
}
### Unit A4 Summary Exercise (about why logic can convey precise meaning)

**
**

While we can discuss a variety of aspects, I will focus on a few essential
ones. First, primitive statements (property and equality) can convey
precise meaning because there is a precise way of interpreting whether an
individual has a property (by checking set membership) and whether an equality
holds (by checking the identities of the referent). Next, complex statements
can convey precise meaning because there are several ways of combining component
statements (primitive or complex), which are syntactically precisely defined.
Each of building a complex statement has a precise rule to interpret the
complex statement out of its component statements. For example, the
truth value of a complex statement that involves "and" depends on the truth
values of both component statements.

In certain cases, the truth values of a statement depends on the context.
In practice, it is very important that we be able to analyze a variety
of contexts to interpret logical statements. For this reason, we tackle
a somewhat challenging exercise (Module A Comprehensive Exercise 2). When
we discuss two standard versions of logic (in Module B), the semantics of
a statement must be interpreted independent of the context.

This summary exercise appears to be more challenging. Although we discussed
all the points described above, not many students were able to point out
the essential points. In this course, we try to prepare ourselves for
the future when we will surely face all sorts of uncertainty. So, we
will do exercises that are not repeating what are shown or said earlier.
We need to be able to analyze (into principles) and synthesize (from
principles) depending on the situation.

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