# 3.E: First Order Logic and Automated Reasoning in a Nutshell (Excercises)

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Exercise $$\PageIndex{1}$$

What is the difference between syntax and semantics for a logic?

Exercise $$\PageIndex{2}$$

What is a theory?

Exercise $$\PageIndex{3}$$

Name the four core components for automated reasoning.

Exercise $$\PageIndex{4}$$

Describe the procedure for tableau reasoning in four shorts sentences.

Exercise $$\PageIndex{1}$$

Write in one natural language sentence what the following sentences in First-Order Logic state.

a. $$\forall x(Lion(x)\to Mammal(x))$$

b. $$\forall x(PC(x)\to\exists y,z(hasPart(x,y)\wedge connected(x,z)\wedge CPU(y)\wedge Monitor(z)))$$

c. $$\forall x,y(hasProperPart(x,y)\to\neg hasProperPart(y,x))$$

(a) All lions are mammals.

(b) Each PC has as part at least one CPU and at least one Monitor connected

(c) Proper part is asymmetric.

Exercise $$\PageIndex{2}$$

Formalize the following natural language sentence into First-Order Logic.

a. Each car is a vehicle.

b. Every human parent has at least one human child.

c. Any person cannot be both a lecturer and a student editor of the same course.

(a) $$\forall x(Car(x)\to Vehicle(x))$$

(b) $$\forall x(HumanParent(x)\to\exists y(haschild(x,y)\wedge Human(y)))$$

(c) $$\forall x,y(Person(x)\wedge Course(y)\to\neg (lecturerOf(x,y)\wedge studentOf(x,y)))$$

Exercise $$\PageIndex{3}$$

Consider the structures in Figure 2.3.2, which are graphs.

a. Figures 2.3.2-A and B are different depictions, but have the same descriptions w.r.t. the vertices and edges. Check this.

Figure 2.3.1: Explanation of the tableaux in Figure 2.2.2.

b. C has a property that A and B do not have. Represent this in a first-order sentence.

c. Find a suitable first-order language for A (/B), and formulate at least two properties of the graph using quantifiers.

Figure 2.3.2: Graphs for Exercise 2.3.3 (figures A-C) and Exercise 2.3.4 (figure D).

(b) There exists a node that does not participate in an instance of $$R$$, or: it does not relate to anything else: $$\exists x\forall y.\neg R(x,y)$$.

(c) $$\mathcal{L} =\langle R \rangle$$ as the binary relation between the vertices. Optionally, on can add the vertices as well. Properties:

$$R$$ is symmetric: $$\forall xy.R(x,y)\to R(y,x)$$.

$$R$$ is irreflexive: $$\forall x.\neg R(x,x)$$.

If you take into account the vertices explicitly, one could say that each note participates in at least two instances of $$R$$ to different nodes.

Exercise $$\PageIndex{4}$$

Consider the graph in Figure 2.3.2, and first-order language $$\mathcal{L} =\langle R\rangle$$, with $$R$$ being a binary relation symbol (edge).

a. Formalize the following properties of the graph as $$\mathcal{L}$$-sentences:

(i) $$(a, a)$$ and $$(b, b)$$ are edges of the graph;

(ii) $$(a, b)$$ is an edge of the graph;

(iii) $$(b, a)$$ is not an edge of the graph. Let $$T$$ stand for the resulting set of sentences.

b. Prove that $$T\cup\{\forall x\forall yR(x,y)\}$$ is unsatisfiable using tableaux calculus.

(a) $$R$$ is reflexive (a thing relates to itself): $$\forall x.R(x,x)$$ ∀x.R(x, x).

$$R$$ is asymmetric (if $$a$$ relates to $$b$$ through relation $$R$$, then $$b$$ does not relate back to $$a$$ through $$R$$): $$\forall xy.R(x,y)\to\neg R(y,x)$$.

Exercise $$\PageIndex{5}$$

Let us have a logical theory $$\Theta$$ with the following sentences:

• $$\forall xPizza(x), \forall xPizzaT(x), \forall xPizzaB(x)$$, which are disjoint
• $$\forall x(Pizza(x) \to\neg PizzaT(x))$$,
• $$\forall x(Pizza(x)\to\neg PizzaB(x))$$,
• $$\forall x(PizzaT(x)\to\neg PizzaB(x))$$,
• $$\forall x,y(hasT(x,y)\to Pizza(x)\wedge PizzaT(y))$$,
• $$\forall x,y(hasB(x,y)\to Pizza(x)\wedge PizzaB(y))$$,
• $$\forall x(ITPizza(x)\to Pizza(x))$$, and
• $$\forall x(ITPizza(x)\to\neg\exists y(hasT(x,y)\wedge FruitT(y))$$, where
• $$\forall x(VegeT(x)\to PizzaT(x))$$ and
• $$\forall x(Fruit(x)\to PizzaT(x))$$.

a. A Pizza margherita has the necessary and sufficient conditions that it has mozzarella, tomato, basilicum and oil as toppings and has a pizza base. Add this to $$\Theta$$. Annotate you commitments: what have you added to $$\Theta$$ and how? Hint: fruits are not vegetables, categorize the toppings, and “necessary and sufficient” is denoted with $$\leftrightarrow$$.
b. We want to merge our new $$\Theta$$ with some other theory $$\Gamma$$ that has knowledge about fruits and vegetables. $$\Gamma$$ contains, among other formulas, $$\forall x(Tomato(x)\to Fruit(x))$$. What happens? Represent the scenario formally, and prove your answer.