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1000 point star (Posted on 2023-10-25) Difficulty: 3 of 5
Typical "stars" are drawn in connected, but not repeated, line segments. For example, a 5-point star is drawn as such - line segments AC, CE, EB, BD, DA, with the order of the points going clockwise being A, B, C, D, and E.

The segments must always skip a constant number of points which is at least one.

Given the information that there is only 1 way to draw a 5-point star, and that there is NO way to draw a 6-point star (in continuous lines, that is), and there are 2 ways to draw a 7-point star.

How many different ways are there to draw a 1000-point star?

See The Solution Submitted by K Sengupta    
Rating: 5.0000 (1 votes)

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Solution Solution | Comment 1 of 6
From the start we see that we are to disregard rotations and reflections during the drawing process, otherwise there would be 10 ways of drawing a 5-pointed star (5 starting vertices * 2 possible "directions" for the first segment).

Next, some labeling. For an n-pointed star, label the vertices of an n-gon 0, 1, 2...n-1. If we start at 0, we can then describe any star by the pair (n, s), where s is the label that corresponds to the first vertex we connect to. For example, in this system a five-pointed star is represented as (5, 2), and traverses the vertices in the order 0, 2, 4, 1, 3, (0).

From the above, our stars are really just the sequences 0, s, 2s...(n-1)s, where the values are taken modulo n. In order to be a valid star, however, none of the terms in this sequence can be equal to each other, since this would represent a repeated vertex in our drawing. This is equivalent to placing the condition that n and s be relatively prime. Lastly, since reflections are to be ignored, our final total must be divided by 2 since each star (n, s) will have an equivalent star (n, (n-s)).

Since 1000 = 2^3 * 5^3, we must exclude all multiples of 2 (500) and all multiples of 5 (200). This double counts all multiples of 10 (100), however, so the final count of pairs (n, s) that will not result in repeated vertices is (1000-500-200+100)/2 = 200. Finally, we must also exclude the pair (1000, 1), since this corresponds to drawing a 1000-gon, not a star. So the total number of possible 1000-point stars is 199.


  Posted by H M on 2023-10-25 07:54:30
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