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So I decided to give a shot on the #453 project euler problem but there is something that confuses me with the numbers given. I decided to start by calculating the possible arrangements of 4 vertices in the 9 vertex grid. To do that I used the binomial coefficient which gives (for the first example) 126 different arrangements. I then removed all non valid arrangements (polygons with straight lines) but the result is smaller than what described. I am probably doing something wrong and I can't figure out what is the issue. Any help is much appreciated!

Edit: Here is the link

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You are missing the fact that some arrangements of four vertices can produce more than one polygon. Eg {(0,0), (0,2), (1,1) ,(1,2)}

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  • $\begingroup$ I can't believe I missed that. Now I wonder if I'm going the right way about this. $\endgroup$ – Veritas Feb 2 '14 at 17:07
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    $\begingroup$ @Veritas That's much more difficult to say :-) $\endgroup$ – leonbloy Feb 2 '14 at 17:11

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