Stacking rings on fingers by decreasing stack size (permutations)

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SUMMARY

The discussion focuses on calculating the permutations of stacking rings on fingers, specifically using the formula for permutations denoted as ##nPk##. When stacking 5 rings, the total configurations are calculated as ##5P5##, and for arranging these on fingers, it is ##5P1##. The analysis extends to configurations with 4 rings, leading to the equation $$x=5P5(5P3+5P2+5P1)$$. The discussion also explores various cases for fewer rings, ultimately refining the approach to consider the arrangement of objects on fingers without initially specifying the number of rings in each stack.

PREREQUISITES
  • Understanding of permutations, specifically the notation ##nPk##.
  • Basic combinatorial principles related to arranging objects.
  • Familiarity with factorial calculations and their applications in permutations.
  • Knowledge of how to apply combinatorial logic to real-world scenarios, such as stacking and arranging items.
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  • Study the concept of permutations in depth, focusing on the formula ##nPk## and its applications.
  • Learn about combinatorial proofs and their relevance in solving stacking problems.
  • Explore variations of stacking problems, such as stacking with restrictions or additional objects.
  • Investigate advanced combinatorial techniques, including generating functions and their use in counting arrangements.
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Mathematicians, educators, students studying combinatorics, and anyone interested in solving complex stacking and arrangement problems using permutations.

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Homework Statement
Let ##x## be the numbers of ways that a woman can wear ##5## distinct rings on ##5## fingers of her right hand, given that she can stack maximum of ##3## rings on any finger.
Relevant Equations
N/A
When she is stacking ##5## rings, then there are ##5P5## configurations when it comes to arranging rings, and each configuration can be arranged on her fingers in ##5P1## ways (choose one finger from 5 to put that configuration on).

When she is stacking ##4## rings, then we have two objects; a configuration of ##4## rings, and a ring outside. The number of configurations is also given by ##5P5##. For the fingers, however, we now choose two fingers out of five to put the objects on, or, ##5P2##.

The same reasoning goes for the last case, so$$x=5P5(5P3+5P2+5P1)$$


EDIT: I understood the problem statement in a peculiar way. I don't know how that happened. However, the logic is kind of still the same, and I think$$x=5P5(5P5+5P4+5P3)$$
 
Last edited:
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I didn't consider the cases where we can stack 2 rings on 2 fingers, for example, etc ..

To make it easier on myself, I can first ignore how many rings are making a specific stack, and just consider objects to be arranged on the five fingers. For example, I'll just look at a configuration of ##2, 2, 1## as just ##3## objects to be arranged.

The number of objects I can possible have is ##5, 4, 3## and ##2##, so ##x=5P5a+5P4b+5P3c+5P2d##, where those variables need to be determined.

When we have ##5## objects, this means that I have a ring per finger, so ##a=1##.

When we have ##4## objects, this means that there is a stack of ##2## and ##3## single rings. The number of ways I can gather a stack of ##2## is ##5P2##, so ##b=5P2##.

When we have ##3## objects, then we either have a stack of ##3## rings, and ##2## single rings, or ##2## stacks of ##2## rings, and single ring.
For the case where we have a stack of ##3##, then this is similar to the above case, i.e we have ##5P3## ways of arranging that stack.
Basically, I can represent one configuration of the set of objects to be permuted by ##\{(r_ar_b),(r_cr_d),r_e\}##. At first, I have ##5P2## ways of choosing ##(r_ar_b)##, then I'd be left with ##3## rings, so I can choose ##(r_cr_d)## in ##3P2## ways. Moreover, I'll be doing twice the same permutation eventually since I'd have to permute ##\{(r_ar_b),(r_cr_d),r_e\}## and ##\{(r_cr_d),(r_ar_b),r_e\}##.
Finally, I get ##c=5P3+\frac{5P2\times3P2}{2}##.

When we have ##2## objects, then we either have ##4,1##, or ##3, 2##.
The first case is again like the others, so ##5P4## permutations.
For ##3, 2##, it is like the last case. We have ##\{(r_ar_br_c), (r_dr_e)\}##, so ##5P3\times2P2##.
Thus ##d=5P4+5P3\times2P2##.
 
Last edited:

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