Search results
Results from the WOW.Com Content Network
The number of k-combinations for all k is the number of subsets of a set of n elements. There are several ways to see that this number is 2 n. In terms of combinations, () =, which is the sum of the nth row (counting from 0) of the binomial coefficients in Pascal's triangle.
Finding the number N, using the formula above, from the k-combination (c k, ..., c 2, c 1) is also known as "ranking", and the opposite operation (given by the greedy algorithm) as "unranking"; the operations are known by these names in most computer algebra systems, and in computational mathematics. [2] [3]
The three-choose-two combination yields two results, depending on whether a bin is allowed to have zero items. In both results the number of bins is 3. If zero is not allowed, the number of cookies should be n = 6, as described in the previous figure. If zero is allowed, the number of cookies should only be n = 3.
The same argument shows that the number of compositions of n into exactly k parts (a k-composition) is given by the binomial coefficient (). Note that by summing over all possible numbers of parts we recover 2 n −1 as the total number of compositions of n :
Sloping lines denote graphs of 2x+5y=n where n is the total in pence, and x and y are the non-negative number of 2p and 5p coins, respectively. A point on a line gives a combination of 2p and 5p for its given total (green). Multiple points on a line imply multiple possible combinations (blue). Only lines with n = 1 or 3 have no points (red).
The number of derangements of a set of size n is known as the subfactorial of n or the n th derangement number or n th de Montmort number (after Pierre Remond de Montmort). Notations for subfactorials in common use include !n, D n, d n, or n¡ . [a] [1] [2] For n > 0 , the subfactorial !n equals the nearest integer to n!/e, where n!
Two of the problems are trivial (the number of equivalence classes is 0 or 1), five problems have an answer in terms of a multiplicative formula of n and x, and the remaining five problems have an answer in terms of combinatorial functions (Stirling numbers and the partition function for a given number of parts).
Combinatorics is an area of mathematics primarily concerned with counting, both as a means and as an end to obtaining results, and certain properties of finite structures.It is closely related to many other areas of mathematics and has many applications ranging from logic to statistical physics and from evolutionary biology to computer science.