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A partial order ≤ is defined on the set S = {x, a1, a2, ... an, y} as x ≤ ai for all i and ai ≤ y for all i, where n ≥ 1. The number of total orders on the set S which contain the partial order ≤ is

  1. n!
  2. n+2
  3. n
  4. 1


asked in Set Theory & Algebra by Veteran (58.9k points)   | 655 views

1 Answer

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Best answer

To make this partial order a total order, we need the relation to hold for every two element of the partial order. Currently between any ai and aj, there is no relation. So, for every ai, aj, we have to add either (ai, aj) or (aj, ai) in total order. So, this translates to giving an ordering for n elements between x and y, which can be done in n! ways. So, answer is (a).

partial order

The bottom figure is for a total order. We can permute the ai from i = 1 to n, and each permutation will also be a total order containing the given partial order. 

answered by Veteran (285k points)  
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Answer to this is (C) i.e n. An order is set in which every two elements are comparable. So we can not take two or more than two ai in a TOSET as you have shown above. Tosets that are possible will be {x,a1,y},{x,a2,y}........{x,an,y}. So only 'n' tosets are possible.

To back up this argument we can also state a qoute from wikipedia that "maximum number of chains that an order can be divided into is ATMOST k, where k is the width of the lattice which here is n.
The question asks for NUMBER of total orders possible. {x, a1, a2, ... an, y} where there element at position i ≤ element at position i+1, is one such TOSET. Similarly, we can have {x, a2, a1, a3, ... an, y} which is another TOSET with the same relation. Both of these TOSET contains the given POSET in question. In this way we can permute in any order and each permutation will be a new TOSET and will contain the given POSET.
OK I got it..I was instead dividing the POSET into maximum number of TOSETS that it can be divided into with the same relation.

But question asks for TOSETs that contain the given POSET in it.

Thank You..
arjun sir,what is meant by the TOSET will contain the given poset?? how is it retained in the TOSET??Can you explain??
@Arjun. Its not that we have to order it in chain. Its that in how many ways we can have x1, x2......,xn arranged in between 'a' and 'b' so that the given partial order is preserved.
@Arjun Sir, I think the given ans fails for subset operation on Powerset of set A={a,b}.

where <= means subset operation


Here x=Phi a1={a} a2={b} y=A

we know {a} is not subset of {b} and vice versa. so both can't present in TOSET.

But u told in ur ans...a1 and a2 both can present simultaneously in n! order..which contradicts with above example.

let A={0,1,2,3,∞}

here x=0, y=∞, a1=1, a2=2, a3=3.

∞                                      ∞

|                                        |

1                and                 3

|                                        |

2                                       1

|                                         |

3                                       2

|                                        |

0                                       0

is a Toset                       is also a Toset 

bcz as mentiond in question , ≤ is applicable only for (x,a1) and (a3,y), however a1, a2, a(1,2,3) can be permutate in any order (3!=6) as long as x has leat value and y has greatest value in the set. same thing applicable for your query, i think. 

In question it is given that

x ≤ ai for all i and ai ≤ y for all i, 

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