proof that a stable matching in the roommate's problem is not guaranteed when there are even numbers

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I came up with the following:

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I don't know if I have proven this appropriately. A friend of mine told me I had to match C with D and from there prove that there is a blocking pair. I'm not sure which way is the correct way.

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The approach using Irving's algorithm should be fine to prove no stable matching exists. If during phase one of the algorithm any participant is eventually rejected by all other participants, this indicates that no stable matching is possible. For the rankings in your problem, by Irving's algorithm we have

  • A proposes to B, B accepts.
  • B proposes to C, C accepts.
  • C proposes to A, A accepts.
  • D proposes to A, A rejects.
  • D next proposes to B, B rejects.
  • D finally proposes to C, C rejects.

D has been rejected by all other participants and no stable matching is possible.

We can also reason about it by saying in any solution A, B, or C must be paired with D. For anyone who is with D, another person will have rated them highest, and D’s partner will prefer this other person over D. Also note that D is last on every persons preferences.