## 03/16/2019 Note: This version will be retired November 2019. Please use Calculator 2.0 instead.

Permutations and Combinations are described on many web sites (e.g., The Math Forum). This article focuses on permutations and combinations with duplicates (multiples) -- as compared to unrestricted repetitions. The audience is high school mathematics students or first year college algebra students.

First, the differences between duplicates and unrestricted repetitions for the purposes here. An example of permutations with unrestricted repetitions is assigning letters and numbers to automobile license plates. For three letters and three digits in the English alphabet, the number of permutations is
26 x 26 x 26 x 10 x 10 x 10 = 17,576,000
since any letter or digit can be selected more than once.

An example of permutations with duplicates is arranging the letters in a word, such as DAFFODIL. If all (eight) letter permutations is the goal, the number of permutations is
8! / (2! x 2!) = 10,080
However, if the count of four letter permutations is the goal, the answer is not
P(8, 4) / (2! x 2!) = 420 incorrect
The actual answer is 606
A simple counterexample to the formula above is the count of two permutations with duplicates of the letters in the word BABY.
Simple manual enumeration yields: AB, AY, BA, BB, BY, YA, YB -- seven total. The formula P(4, 2) / 2! yields 6, which is not the correct answer. This is the primary reason for this article -- in order to dispel this misconception.
The solution involves enumerating generating functions -- beyond the scope of high school algebra textbooks.
References: "Introduction to Combinatorial Analysis" by John Riordan, though he doesn't use the duplicate terminology used here. In combinatorial analysis these are now called permutations of multisets. See also The Math Forum - Four-Letter Combinations.
(Though the formula P(8, 7) / (2! x 2!) does work for seven letter permutations of DAFFODIL also which gives the false impression that this is a general formula.)
This column has links to three online calculators that provide the correct answers for such permutations and combinations.

1. Online Permutations Combinations Calculator - Word version that optionally handles duplicates.
For example, select Permutations and enter for the word BABY and 2 for the 'Number of characters.' The number of two letter permutations of the word BABY will be displayed.
Another example, select Permutations and enter for the word DAFFODIL and 4 for the 'Number of characters.' The number of four letter permutations of the word will be displayed.

2. Online Permutations Combinations Calculator - General version that optionally handles duplicates.
Standard notation P(N, R) and C(N, R) is for N objects taken R at a time.
For example, select Permutations and enter for N 4, for R 2, and for List Counts of Duplicates 2 (two B's). The answer will be 7 -- as for the two letter permutations of the word BABY.
Another example, select Permutations and enter for N 8, for R 4, and for List Counts of Duplicates 2 2. The answer will be 606 -- as for the four letter permutations of the word DAFFODIL. There are two letters for the group D,D and two letters for the group F,F.
Notation for the calculator:
p1 + p2 + ... + pk = N
where pj is the count of duplicate objects in a group.
pj is assumed to be one for all j unless specified otherwise.

3. Online Permutations Combinations Generator
For example, select Permutations and enter for the word BABY and 2 for the 'Number of characters.' The seven two letter permutations of the word BABY will be listed.
Another example, select Permutations and enter for the word DAFFODIL and 4 for the 'Number of characters.' The 606 four letter permutations of the word will be listed.
The generation output limit is 6,000. 
Credits: The online book "Combinatorial Generation" by Frank Ruskey (2003) As of March 13, 2019 available on SCRIBD. Section 4.5 "Permutations of a Multiset" page 69; Section 4.5.1 "Combinations of a Multiset" page 71. Algorithms to generate Permutations and Combinations.

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