The closed question was merged into the non-closed question.
This QuestionQuestion got closed as a duplicate of another by the same user, namely thisnamely this, but it is the former that has an Answer. (The former was migrated to Math.SE as the new user had cross-posted the same problem.)
I recommend the closed-as-duplicate status be switched to the unanswered version, and have voted to reopen accordingly (so the other/unanswered one can be closed instead).