Your question is phrased as an isolated problem, without any further information or context. This does not match many users' quality standards, so it may attract downvotes, or be put on hold. To prevent that, please edit the question. This will help you recognise and resolve the issues. Concretely: please provide context, and include your work and thoughts on the problem. These changes can help in formulating more appropriate answers.

@Martin This question is perfectly clear and requires no additional context. What it requires is a full solution. Why must the OP include irrelevant details about what they've tried?

@MattSamuel We do not require more context to solve the problem. We do require more context to consider this a good post worth investing time and effort into helping.

@MattSamuel Thanks friend! ))

@MattSamuel The question is clear but the level of effort is not.

@Arthur If it's not worth your time, by all means don't answer it. What boggles my mind is that you think it's worth your time *to prevent other people from answering it. *

@john-douma The level is high. Olympiad on mathematics. )

@JoeBradley I see that statement to be one of the biggest problems on this site. Everyone agrees that a question like "how do I find the equation of the line with slope $1$ through $(1,0)$?" with no effort shown should be closed. On the other hand, if I ask to show that the free group functor is left adjoint to the forgetful functor with no effort, I may receive $10$ or more upvotes.

@MattSamuel We don't want this kind of posts on our site. We want to discourage people from asking such questions. One of the strongest tools we have to this end is to remove such questions and make sure the asker isn't rewarded for posting it, as well as other potential askers seeing that it won't pay off. The process is flawed in its inconsistency, but it is what we have.

@john-douma I don't see any problem with the statement. The category theory specialist is sure to solve this one. )) I mean Olympiad for 16-year-olds.

@Arthur Who are "we" other than the collective users of the site - so some disagree. The context here is someone who is trying to help a student and has a problem beyond their own apparent ability. The ideal answer would indicate a good hint which would help OP to answer the question and also to give their own student a good hint.

@MarkBennet The context was added after this discussion started. Also, this is is the official stance of the community: See for instance this or this.

Have you tried with smaller odd numbers/primes to see if you can spot any pattern. What happens with $5$? With $7$?

@arthur Hopeful mathematics student with some competition level problem solving experience doesn't like my context about the math competition problem? Why?

@Mark Bennet Thank for supporting me. ) Unfortunately, the situation for 9, 13, 15, 20+ and other sequence lengths is different. (

@Matt Samuel I know Matt. That's why I wrote "The level is high". )

@JoeBradley Are you sure such a sequence exists? Which Math Olympia had this as a problem? They usually publish their solutions.

@JoeBradley There is a difference between liking the problem and liking the post. Take a look at this page to get an idea of what you could add to make me like the post a lot more.

@Arthur I didn't write about liking the problem. I wrote about liking the context "I'm a math teacher and my student brought this task. But I can't solve it without coding. (("

@john-douma I would be glad if this task had been given at any concrete competition. ) Тhis is the final task of one of the training test to prepare for the USE in Russia. The complexity of this task traditionally corresponds to the level of a good math Olympiad. If you like I can provide a link to this test (in Russian of course). There's only answer there, no solution.