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If the longest chain in a partial order is of length $n$, then the partial order can be written as a _____ of $n$ antichains.

Suppose the length of the longest chain in a partial order is n. Then the elements in the poset can be partitioned into n disjoint antichains.
could you please explain with an example ?
@Arjun Sir ...not getting this Q/A... antichain  ??

If the longest chain in a partial order is of length n, then the partial order can be written as a Partition of n antichains.

each element of chain must be in different antichains so if chain having n elements so there must exist n disjoint antichains .

​​​​Reference for more details.

CHAIN : Let (A ,⋨) be a Poset .A subset of A is called Chain if every two elements in the subset are related.

ANTICHAIN: A subset of A is called Antichain if no two distinct elements in the subset are related.

THEOREM:  Let (A ,⋨) be a Poset.Suppose the length of the longest chains in A is n.Then the elements in A can be partitioned into n disjoint Antichains.

Ex : number of anti chains in [P(A),subset] where A = {1,2} is 3 (i.e)

1) {} ,

2) {1,2} ,

3) {1} and {2} as {1} and {2} are not related they can form a single anti-chain.