Problem 2 Let A be a set, and let R be a relation on A. Suppose that R is symmetric and transitive. Find the flaw in the following alleged proof that this relation is necessarily reflexive. “Let x ∈A. Choose y ∈A such that xRy. By symmetry know that yRx, and then by transitivity we see that xRx. Hence R is reflexive.”

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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Problem 2
Let A be a set, and let R be a relation on A. Suppose that R is symmetric and transitive.
Find the flaw in the following alleged proof that this relation is necessarily reflexive. “Let
x ∈A. Choose y ∈A such that xRy. By symmetry know that yRx, and then by transitivity
we see that xRx. Hence R is reflexive.”

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