## Most recent change of BinomialCoefficient

Edit made on July 10, 2009 by GuestEditor at 10:12:24

Deleted text in red / Inserted text in green

WW
Binomial coefficients can be defined in three equivalent ways. EQN:{n\choose{}r} is ...

* ... the coefficient of EQN:x^r when you expand out EQN:(1+x)^n . (This is where the name comes from.)
* ... the number of different ways to select /r/ objects from a set of /n./ (This is why EQN:n\choose~r is sometimes pronounced "n choose r".) (See combinations)
* ... defined by the following three facts:
** (1) EQN:n\choose~r is 0 whenever EQN:r<0 or EQN:r>n
** (2) EQN:n\choose~r is 1 whenever EQN:r=0 or EQN:r=n
** (3) EQN:{n+1\choose~r+1}={n\choose~r}+{n\choose~r+1}
*** (These are the equations that define Pascal's triangle.)

The interplay between these three quite different ways of thinking about the same objects leads to a great deal of beautiful mathematics.

The binomial coefficients turn up (unsurprisingly) in the Binomial Theorem.

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!! Enrichment
Show that the "three facts" given above can be reduced to these two:
* (1) EQN:0\choose~r is 1 if and only if /r=0/ and 0 otherwise
* (2) EQN:{n+1\choose~r+1}={n\choose~r}+{n\choose~r+1}