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- Intro to logic
- First-order logic
- Exercises for first-order logic
Throughout these exercises,
is simply a shorthand for
.
Relations and interpretations
Consider the binary relation
on the domain
.
-
List all the ordered pairs in the relation.
-
Display the relation as a directed graph.
-
Display the relation in tabular form.
-
Is the relation reflexive? symmetric? transitive?
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How would you define
as a ternary relation?
- Give a prose definition of
in terms of the addition function.
-
List the set of triples in the relation on the domain
.
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Are each of the following formulas valid,
i.e. , true for all interpretations?
(Remember that the relation names are just names in the formula;don't assume the name has to have any bearing on their interpretation.)
- For arbitrary
and
in the domain,
- For arbitrary
in the domain,
- For arbitrary
and
in the domain,
For each, if it is true or false under all interpretations, prove that.
For these small examples, a truth table like
this one will probably be easier
than using Boolean algebra or inference rules.Otherwise, give an interpretation in which it is true,
and one in which it is false.
As always, look at trivial and small test cases first.
Here, try domains with zero, one, or two elements,and small relations.
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[Practice problem
solution provided.]
Suppose we wanted to represent the count of neighboring pirates
with a binary relation, such that when location
has two neighboring pirates,
will be true. Of course,
would not be true in this situation.
These would be analogous with the propositional WaterWorld propositions
and
, respectively.
-
If we only allow binary relations to be subsets of
a domain crossed with itself,then what must the domain be
for this new relation
?
-
If we further introduced another relation,
,
what is a formula that would help distinguishintended interpretations from unintended interpretations?
That is, give a formula that is true under all our intendedinterpretations of
but is not true for some
nonsense
interpretations we want to exclude.
(This will be a formula without an analog in the
WaterWorld domain axioms .)
-
The relation needs to accept locations as well as numbers,
so the domain is
,
where
is the set of WaterWorld locations. Alternatively, you could use
instead of
,
the set of all natural numbers.
-
The difficulty is that it's possible to ask about
nonsensical combinations like
and
.
Adding
,
any interpretation would be expected to satisfy, for arbitrary
and
,
.
More interestingly though, imagine we did interpret
over the domain
only.
We could then pretend that the locations, instead of being named
,…,
,
were just numbered 1,…,24.While this representation doesn't reflect how we model the problem,
it is legal.Exercise for the reader: Write a formula which excludes relation
which can't match this convention!
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Source:
OpenStax, Intro to logic. OpenStax CNX. Jan 29, 2008 Download for free at http://cnx.org/content/col10154/1.20
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