Do you mean prime factors? If so, that’s definitely wrong. If not, 630 is a factor of 1260.
I think I get what you mean. So you would include 2^6 as well in the 2^12 example, right?
The first question last year was, in my opinion, easier than both of the questions this year. It was definitely easier to qualify though, because the cutoff was only one question.
Oh yeah you are right, I was wrong in that comment. Wow two people upvoted that lol.
The answer is:
2^12 - ((2^1) + (2^2 - 2^1) + (2^4 - (2^2 - 2^1) - 2^1) + (2^3 - 2^1) + (2^6 - (2^3 - 2^1) - (2^2 - 2^1) + 2^1))
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On the other hand, we’ve better programmers appearing this year…
I did take that into account, I still think it was harder last year.
according to me last year’s first question was definitely harder then the second question this year and It totally depends on the person whether or not it was harder then the first question this year. last year’s recurrence was easy where as coding part was difficult whereas it was opposite in this year’s first problem
@superty, today being the day I spend drowned in anxiety as to whether I’d qualify, I can only hope that’s true and I qualify. 
I disagree that last year’s recurrence was easy, I would argue that the recurrence was difficult but the implementation was easy. This year both were hard… oh. Yeah maybe this year was harder. One would expect it to get harder as time goes on anyways.
I’m just wondering, do they actually check the code, or just judge with testcases? I came up with an algorithm for the second question, similar to @sandy999’s but I was anxious about the O(N^2) complexity (and it was terribly slow, was taking more than 3 seconds for an n=150000 input, which was probably some flaw in my implementation, considering that it worked for some others), so I precomputed the number of non-periodic strings for all prime numbers till 387 and put them in a lookup table. I’m hoping precomputation doesn’t count against my score, since the code was working fine for all sample cases.
(Now that I’ve a physical keyboard to answer with.)
I started with the second question, roughly following the same track as @superty. For the first question, I quickly coded the solution for subtask 3, being as it was the easiest with maximum marks. Then I started thinking of a “complete” solution, and once I realized the track, started implementing it.
I couldn’t do that in time though, so I got 130 total. I was silly because I didn’t realize I can solve subtasks 1,2,3 by checking which of the assumption holds true. A few of my friends did that, scoring 160. Hope that’s not the cutoff.
I don’t think that’s officially wrong (no rule says you can’t pre-compute). You’ll want to mail the organisers to be double-sure. It’s still somewhat a “bad” way to solve the problem, given that you obviously did something wrong.
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I intended to do subtasks 1, 2, 3 that way, but I got the full solution after I did the first subtask and then I implemented the complete solution.
After that I went back to the second problem and did the supposedly O(N^2) solution.
Heh; that didn’t even strike me as a possibility during the exam. I’m not sure what to think of myself.
Well, we all overlook things. I don’t know.
Any clue when the results are supposed to be out?
I asked—in a week.
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Thank you.
Given that we’ve more or less a week to the results, does everyone want to participate in a quick collaboration to figure out the cut-off?
Just simply note in a comment the number of people you think will score >= 100, including yourself. Please don’t give answers with duplicates.
I know of myself scoring (probably) 130. (A few more, but those are guessimates.)
Cheers.
I’m the only person I know of, so that makes 2 + the few people you know of getting ~160.
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How many of you are getting above 110, please comment.
Hope not many 
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