Post a question whose answer is what you want to write; e.g. methods for computing modular exponentation were posted as answers to How do I compute $a^b\,\bmod c$ by hand?
If I were to post a question with the intent of having it available to link to from other answers, I would say something to that effect as an addendum or in a comment; e.g. Is it possible that every set can be specified?
That said, it's not clear to me whether it's appropriate to use m.s.e as a publishing platform for things one finds interesting. IIRC, there have been mixed opinions on this practice in the past. Recall from the tour
Focus on questions about an actual problem you have faced.
And the advocate for self-answered questions from a over half-a-decade ago (which may not apply to today's attitudes) gives examples that are not simply interesting things, but question-and-answer pairs that reflect actual problems the author faced and the answer they worked out on their own.