Combinations
https://www.youtube.com/watch?v=0iMtlus-afo&list=TLPQMjQxMDIwMjBfeWCl0eYdng&index=30 https://en.wikipedia.org/wiki/Pascal%27s_triangle https://en.wikipedia.org/wiki/Binomial_theorem https://en.wikipedia.org/wiki/Binomial_distribution https://rosettacode.org/wiki/Combinations_and_permutations https://en.wikipedia.org/wiki/Combination https://en.wikipedia.org/wiki/Permutation https://rosettacode.org/wiki/Gamma_function https://rosettacode.org/wiki/Evaluate_binomial_coefficients https://en.wikipedia.org/wiki/Twelvefold_way https://en.wikipedia.org/wiki/Multiset https://en.wikipedia.org/wiki/Partition_of_a_set https://en.wikipedia.org/wiki/Partition_(number_theory)
This formula shows that the number of ways to select 3 elements from the 10-element set is the same as the number of ways to select 7:
Combinations, Pascal's triangle, Powers of 2, Powerset, Binomial coefficient
In combinations, the order in which we select objects is irrelevant. We're only interested in the subsets that we can form out of a given set and/or number of subsets.
If a given set A contains 5 elements, A = {a,b,c,d,e}, we might as well imagine the set A as an opaque bag that contains 5 marbles (labelled with letters a-e). Then, we might be interested, for example, in finding out the number of ways we can pick 3 distinct elements from A (with the order in which the individual elements are picked is immaterial).
Partitions
https://en.wikipedia.org/wiki/Partition_(number_theory) https://artofproblemsolving.com/wiki/index.php/Partition_(combinatorics)
In combinatorics, C(n,r)
, since r must be 0 <= r <= n
, we can consider all the cases of C(n,r[i])
where i
iterates from 0
to n
.
For example, C(3,r)
can be partitioned into 4 partitions: C(3,0)
, C(3,1)
, C(3,2)
, C(3,3)
. Each partition has a certain frequency.
Adding these frequencies together we get the total number of partitions possible for a set of n
elements (with variable r
i.e. variable partition size): C(3,0)
=1 + C(3,1)
=3 + C(3,2)
=3 + C(3,3)
=1 => 8
A particular partition frequency can be divided by the number of all possible partitions to get the ratio of those combos accounted for by that particular partition. E.g. C(3,1)=3, so its frequency ratio is 3/8. Adding all freq. ratios amounts to 1. E.g. 1/8 + 3/8 + 3/8 + 1+8 = 1
Examples
List all 2-element partitions of a 4-element set
The number of 2-element partitions: C(4,2) = 4!/2!2! = 12/2 = 6
Let set A={a,b,c,d}, then there are 6 two-element subsets of P(A): ab, ac, ad, bc, bd, cd (incorrect but convenient notation)
How many ways to choose 2 out of 4
How may distinct pairs can be made with 4 elements
Check: X={a,b,c,d} n=4 6 pairs: ab, ac, ad, bc, bd, cd
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