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Cool Count (Posted on 2005-08-10) Difficulty: 3 of 5
The binomial coefficients T(n)=n(n+1)/2 are called triangular numbers because T(n) points can be arranged into a triangular array with n points on a side. For example, T(4)=10 points can be arranged in the familiar pattern of bowling pins. Call this triangular array with n points on a side Array(n).

If we count the number of triples of points in Array(n) which are vertices of equilateral triangles with sides parallel to those of the whole array, we get the binomial coefficient (n+1)n(n-1)/6: a nice formula in closed form (i.e., no sum of stuff).

What surprised me when I dared look at it is that if we count the number E(n) of ALL triples of points in Array(n) which are vertices of equilateral triangles, we also get a nice formula.

What is that nice formula for E(n)?

No Solution Yet Submitted by McWorter    
Rating: 4.1429 (7 votes)

Comments: ( You must be logged in to post comments.)
  Subject Author Date
re(5): Not a nice formulaMcWorter2005-08-12 03:30:53
Some Thoughtsre(4): Not a nice formulaBob Smith2005-08-12 02:00:24
re(3): Not a nice formulaMcWorter2005-08-11 15:10:37
Solutionre(2): Not a nice formulaBob Smith2005-08-11 13:14:13
re: closed form solutionMcWorter2005-08-11 00:11:52
re: Not a nice formulaMcWorter2005-08-10 23:59:23
Some Thoughtsclosed form solutionJosh706792005-08-10 20:35:06
Some ThoughtsNot a nice formulaBob Smith2005-08-10 19:16:50
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