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## Homework Statement

Figure out a self-referential formula for the number of handshakes required for a group of n aliens to introduce themselves by hand-calculating a few small values and coming up with a solution.

## Homework Equations

We are given:

Let H(n) be the number of handshakes for a group of n aliens to introduce themselves. Then H(n) = [n(n-1)(n-2)]/6

## The Attempt at a Solution

I couldn't find anything in my textbook about how to derive a self-referential formula. I tried hand-calculating a few small values for H(n) by substituting n for 1, 2, 3, 4, 5 and 6.

H(1) = 0

H(2) = 0

H(3) = 1

H(4) = 4

H(5) = 10

H(6) = 20

I'm not sure what to do next. How do I derive a self-referential formula from this information?