If you are so sure that you want to pursue these subjects, I am not sure why you would want to apply undecided. On the other hand, if you are still not sure this is the path you want to pursue, you know that you can still change your major AFTER you are in the university. The advantage of declaring your major is that it gives you the opportunity to talk about your love for the subject, how you developed your interest, what have you done so far to foster your interest, what you plan to do with the course in the future and so on. But if you have several other courses that you are equally interested in, you could then talk in your essay about how you have several areas of learning that are equally appealing to you and why you are not sure what exactly you what you would like to pursue in the future. The thing is to demonstrate your passion for learning, it does not matter in which area. I don't think applying undecided will hurt your application. It is how well you present yourself that will swing the vote in your favour. I know of many students who have applied UNDECIDED who have got into top schools including the Ivies, so that is not an issue. Hope this helps.
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