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Agata [3.3K]
3 years ago
11

Let A = {a, b, c, d}.

Mathematics
1 answer:
Marta_Voda [28]3 years ago
3 0

Answer:

a. 16

b. 24

c.  9

d. 256

Step-by-step explanation:

Let cardinality of the set A (=|A|) = n.

Any subset of A can contain i elements. i = 0,1,\ldots,n

Now these i elements can be chosen in \binom{n}{i} ways. So the number of subsets can be written as,

\binom{n}{0} + \binom{n}{1} + \dotsc + \binom{n}{n} = 2^n

a. Here we have n = 4. So the Total no. of subsets = 2^4 = 16.

b. The no. of permutations is n! for any set with cardinality n. So, here it is = 4! = 24

c. Let A_i denote the set consisting of all permutations of A where i is fixed, i = 1,\dotsc,4. Using symmetry, |A_i| = 3!(fix one element and permute the rest) is the same \forall \; i =1,2,3,4. Also |A_i \cap A_j| = 2! (fix 2 elements and permute the rest).

By similar arguments, |A_i \cap A_j \cap A_k| = 1 and |\bigcap_{i} A_i| = 1.

Recall the Principle of inclusion exclusion,

|A_1 \cup A_2 \cup \dotsc A_n| = \sum_{i=1}^n |A_i| + \sum_{i < j} |A_i \cap A_j| +  \dotsc + (-1)^{n+1} |\cap_{i}A_i|

Note that \cup_{i=1}^4 A_i = S is the set containing permutations with at least one fixed point. So we require 4! - S.

Computing S.

S = \binom{4}{1} 3! - \binom{4}{2} 2! + \binom{4}{3} 1 - \binom{4}{4} 1 = 15

Required answer is 4! - S = 24 - 15 = 9

d. In general the no.  of functions from A (|A| = n) to B (|B| = m) is given by, m^n. Any element of A can be assigned to any of the m elements in B, so the possibilities are m \times m \times \dotsc n\; times \; = m^n.

Here m = n = 4. So the answer is 4^4 = 256.

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