Chapter 1: Problem 25
The game of bridge is played by 4 players, each of whom is dealt 13 cards. How many bridge deals are possible?
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Chapter 1: Problem 25
The game of bridge is played by 4 players, each of whom is dealt 13 cards. How many bridge deals are possible?
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An elevator starts at the basement with 8 people (not including the elevator operator) and discharges them all by the time it reaches the top floor, number 6. In how many ways could the operator have perceived the people leaving the elevator if all people look alike to him? What if the 8 people consisted of 5 men and 3 women and the operator could tell a man from a woman?
There are \(\left(\begin{array}{l}n \\ r\end{array}\right)\) different linear arrangements of \(n\) balls of which \(r\) are black and \(n-r\) are white. Give a combinatorial explanation of this fact.
The following identity is known as Fermat's combinatorial identity. $$ \text { - }\left(\begin{array}{l} n \\ k \end{array}\right)=\sum_{i=k}^{n}\left(\begin{array}{l} i-1 \\ k-1 \end{array}\right) \quad n \geq k $$ Give a combinatorial argument (no computations are needed) to establish this identity. HINI: Consider the set of numbers 1 through \(n\). How many subsets of size \(k\) have \(i\) as their highest-numbered member?
Consider \(n\)-digit numbers where each digit is one of the 10 integers 0 , \(1, \ldots, 9 .\) How many such numbers are there for which (a) no two consecutive digits are equal; (b) 0 appears as a digit a total of \(i\) times, \(i=0, \ldots, n\) ?
From a set of \(n\) people a committee of size \(j\) is to be chosen, and from this committee a subcommittee of size \(i, i \leq j\), is also to be chosen. (a) Derive a combinatorial identity by computing, in two ways, the number of possible choices of the committee and subcommittee-first by supposing that the committee is chosen first and then the subcommittee, and second by supposing that the subcommittee is chosen first and then the remaining members of the committee are chosen. (b) Use part (a) to prove the following combinatorial identity: $$ \sum_{j=i}^{n}\left(\begin{array}{l} n \\ j \end{array}\right)\left(\begin{array}{l} j \\ i \end{array}\right)=\left(\begin{array}{l} n \\ i \end{array}\right) 2^{n-i} \quad i \leq n $$ (c) Use part (a) and Theoretical Exercise 13 to show that $$ \sum_{j=i}^{n}\left(\begin{array}{l} n \\ j \end{array}\right)\left(\begin{array}{l} j \\ i \end{array}\right)(-1)^{n-j}=0 \quad i \leq n $$
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