# Formula for the number of connections needed to connect every node in a set?

Assuming you have a set of nodes, how do you determine how many connections are needed to connect every node to every other node in the set?

Example input and output:

In   Out
<=1  0
2    1
3    3
4    6
5    10
6    15

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If there are $n$ nodes, then this is called "$n$ choose $2$", and is equal to the number of $2$-element subsets of a set of $n$ elements. The Wikipedia article on binomial coefficients includes this and generalizations.

Since I started writing you discovered the correct formula. However, if you ever have a similar problem where you are trying to figure out a general form for the terms in a sequence from some initial values, a good tool is The On-Line Encyclopedia of Integer Sequences. In this case, entering 0,1,3,6,10,15 brings up a useful entry in which you can find the general form and references.

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That link to oeis.org looks incredibly useful. I was actually looking for something like that, thanks! – Jake Petroules Jul 18 '11 at 17:55

Here is what you want.$$\sum_{k=1}^{n-1}k=\frac{n(n-1)}{2}$$

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Figured it out. The formula is:

x = n(n - 1) / 2

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