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Consider a directed graph with nodes {1,2,3...n} and include an edge (i,j) whenever i<j.

According to me, it should be n(n-1)/2 but the book says it's nC2(combinations).

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  • $\begingroup$ "whenever inC2 edges" - what was this meant to say? $\endgroup$ Oct 5 '15 at 9:41
  • $\begingroup$ @TomvanderZanden: Formatting is messing things up. Let me correct. $\endgroup$ Oct 5 '15 at 9:54
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Did you try working out what n choose 2 evaluates to?

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  • $\begingroup$ I certainly did not. Got carried away :) $\endgroup$ Oct 5 '15 at 9:58

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