# Unfamiliar notation that looks like a vector but isnt

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I'm trying to get my head around a certain equation (well, actually a few), but they have this notation that I initially thought was a vector with 2 elements, but when I worked through it I found that didnt make sense... Luckily in the paper I was reading it gives the parameters and the answer, but I dont get how they have got from one to the other e.g. (assume the little brackets are big ones)
126 = (9)
(5)

126 = (9)
(4)

84 = (9)
(3)

36 = (9)
(7)


Wierd stuff. Anyone any clues here? Cheers, Dave.

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that's the "choose" function, used in many fields but originating in probability. as an example, lets say you had 9 types of topping for a hot dog, and you wanted to find out how many different combinations you could make by picking 5 of those toppings, 9 choose 5 is your answer, or 126.

Mathworld explanation

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Excellent! Cheers for that, you have no idea how stumped that had me. I know a little about combinatorics, but only ever seen nCk style before, never this damned ambiguous notation!

Thanks again,

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Rutin
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JoeJ
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