In mathematics, the Gaussian binomial coefficients (also called Gaussian coefficients, Gaussian polynomials, or q-binomial coefficients) are q-analogs of the binomial coefficients. The Gaussian binomial coefficient, written as or , is a polynomial in q with integer coefficients, whose value when q is set to a prime power counts the number of subspaces of dimension k in a vector space of dimension n over , a finite field with q elements; i.e. it is the number of points in the finite Grassmannian . Definition The Gaussian binomial coefficients are defined by: where m and r are non-negative integers. If , this evaluates to 0. For , the value is 1 since both the numerator and denominator are empty products. Although the formula at first appears to be a rational function, it actually is a polynomial, because the division is exact in Z[q] All of the factors in numerator and denominator are divisible by , and the quotient is the q-number: Dividing out these factors gives the equivalent formula In terms of the q factorial , the formula can be stated as Substituting into gives the ordinary binomial coefficient . The Gaussian binomial coefficient has finite values as : Examples Combinatorial descriptions Inversions One combinatorial description of Gaussian binomial coefficients involves inversions. The ordinary binomial coefficient counts the -combinations chosen from an -element set. If one takes those elements to be the different character positions in a word of length , then each -combination corresponds to a word of length using an alphabet of two letters, say with copies of the letter 1 (indicating the positions in the chosen combination) and letters 0 (for the remaining positions). So, for example, the words using 0s and 1s are . To obtain the Gaussian binomial coefficient , each word is associated with a factor , where is the number of inversions of the word, where, in this case, an inversion is a pair of positions where the left of the pair holds the letter 1 and the right position holds the letter 0. With the example above, there is one word with 0 inversions, , one word with 1 inversion, , two words with 2 inversions, , , one word with 3 inversions, , and one word with 4 inversions, . This is also the number of left-shifts of the 1s from the initial position. These correspond to the coefficients in . Another way