# Binomial coefficients/Introduction/Section

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Let and denote natural numbers with . Then

*binomial coefficient*choose

One can write this fraction also as

because th factors from are also in . In this representation, we have the same number of factors in the numerator and in the denominator. Sometimes it is useful to allow also negative or and define in these cases the binomial coefficients to be .

From the very definition, it is not immediately clear that the binomial coefficients are natural numbers. This follows from the following relationship.