Question

The set D1D_1 consists of three students: Garth, who is 5 feet 11 inches tall; Erin, who is 5 feet 6 inches tall; and Marty, who is 6 feet tall. The set D2D_2 consists of four students: Dale, who is 5 feet 11 inches tall; Garth, who is 5 feet 11 inches tall; Erin, who is 5 feet 6 inches tall; and Mary, who is 6 feet tall. The set D3D_3 consists of one student: Dale, who is 6 feet tall. The set D4D_4 consists of three students: Part, Sandy, and Gal, each of whom is 5 feet 11 inches tall. T1(x,y)T_1(x,y) is the propositional function "x is taller than y." Tell whether each proposition is true or false if the domain of discourse is Di×DjD_i\times D_j for the given values of i and j.

  1. xyT1(x,y)\forall x\forall yT_1(x,y)
  2. xyT1(x,y)\forall x\exists yT_1(x,y)
  3. xyT1(x,y)\exists x\forall yT_1(x,y)
  4. xyT1(x,y)\exists x\exists yT_1(x,y)

i=4, j=1

Solution

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Given:

T1(x,y)T_1(x,y) =xx is taller than yy

D1D_1={\{Garth (5 ft 11 in), Erin (5 ft 6 in), Marty (6 ft)}\}

D2D_2={\{Dale (6 ft); Garth (5 ft 11 in), Erin (5 ft 6 in), Marty (6 ft)}\}

D3D_3={\{Dale (6 ft)}\}

D4D_4={\{Pat (5 ft 11 in), Sandy (5 ft 11 in), Gale (5 ft 11 in)}\}

i=4i=4

j=1j=1

Cartesian product\textbf{Cartesian product} of AA and BB: A×B={(a,b)aAbB}A\times B=\{(a,b)|a\in A\wedge b\in B\}

Let us determine all elements of Di×DjD_i\times D_j:

D4×D1={(Pat,Garth),(Pat,Erin),(Pat,Marty),(Sandy,Garth),(Sandy,Erin),(Sandy,Marty),(Gale,Garth),(Gale,Erin),(Gale,Marty)}\begin{align*} D_4\times D_1=\{&(Pat, Garth), (Pat, Erin), (Pat, Marty), \\ &(Sandy, Garth), (Sandy, Erin), (Sandy, Marty), \\ &(Gale, Garth), (Gale, Erin), (Gale, Marty)\} \end{align*}

xyT1(x,y)\forall x\forall y T_1(x,y): Everyone is taller than everyone. This statement is false\textbf{false}, because we note that Pat is not taller than Marty.

xyT1(x,y)\forall x\exists y T_1(x,y): Everyone is taller than somebody. This statement is true\textbf{true}, because we note that Pat, Sandy and Gale are all taller than Erin.

xyT1(x,y)\exists x\forall y T_1(x,y): Somebody is taller than everyone. This statement is false\textbf{false}, because we note that Pat, Sandy and Gale are all not taller than Marty.

xyT1(x,y)\exists x\exists y T_1(x,y): Someone is taller than someone. This statement is true\textbf{true}, because we note that Pat is taller than Erin.

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