Set theory?

Calculus Level pending

Let Y X P ( X × Y ) Y^{X} \subset P(X \times Y) denote the set of all functions f : X Y f: X \rightarrow Y . If N N is the number of elements in Y ϕ Y^{\phi} and M M is the number of elements in ϕ X \phi^{X} ( X X is not empty), then what is N + M N + M ?

Details:

  • P ( K ) P(K) denotes the power set of the set K K .
  • ϕ \phi is the empty set.


The answer is 1.

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1 solution

Lucas Tell Marchi
Feb 26, 2014

Obs: x |x| means the set that contains x x . Well, as we know from the problem:

Y ϕ P ( ϕ × Y ) = P ( ϕ ) = ϕ Y ϕ ϕ Y^{\phi} \subset P(\phi \times Y) = P(\phi) = |\phi| \therefore Y^{\phi} \subset |\phi|

So either Y ϕ = ϕ Y^{\phi} = \phi or else Y ϕ = ϕ Y^{\phi} = |\phi| . But f = ϕ f = \phi satisfies f : ϕ Y f: \phi \rightarrow Y , therefore Y ϕ Y^{\phi} has at least one element. But Y ϕ = ϕ Y^{\phi} = |\phi| is the only set that satisfies this condition, therefore N = 1 N = 1 . But now observe that

ϕ X P ( X × ϕ ) = P ( ϕ ) = ϕ ϕ X ϕ \phi^{X} \subset P(X \times \phi) = P(\phi) = |\phi| \therefore \phi^{X} \subset |\phi|

So either ϕ X = ϕ \phi^{X} = |\phi| or else ϕ X = ϕ \phi^{X} = \phi . But if X ϕ X \neq \phi , then for some x X x \in X , there is no y Y y \in Y (because Y = ϕ Y = \phi . Therefore there does not exist f f such that

f : X ϕ ϕ f: X \neq \phi \rightarrow \phi

And then ϕ X = ϕ \phi^{X} = \phi which means that M = 0 M = 0 . We want N + M = 1 + 0 = 1 N + M = 1 + 0 = 1 .

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