Arrangement:

$\quad$ Number of permutations of $n$ different things taken $r$ at a time $=$

${ }^{n} P_{r}=n(n-1)(n-2) \ldots(n-r+1)=\frac{n !}{(n-r) !}$

Circular permutation :

$\quad$ The number of circular permutations of $n$ different things taken all at a time is; $(n-1)$ !

Selection :

$\quad$ Number of combinations of $n$ different things taken $r$ at a time

$={ }^{n} C_{r}=\frac{n !}{r !(n-r) !}=\frac{{ }^{n} P_{r}}{r !}$

$\quad$ The number of permutations of ’ $n$ ’ things, taken all at a time, when ’ $p$ ’ of them are similar & of one type, $q$ of them are similar & of another type,

$\quad$ ’ $r$ ’ of them are similar & of a third type & the remaining $n-(p+q+r)$ are all different is

$\frac{n !}{p ! q ! r !}$

Selection of one or more objects:

• Number of ways in which atleast one object be selected out of ’ $n$ ’ distinct objects is

$ { }^{n} C_{1}+{ }^{n} C_{2}+{ }^{n} C_{3}+\ldots \ldots \ldots \ldots \ldots+{ }^{n} C_{n}=2^{n}-1 $

• Number of ways in which atleast one object may be selected out of ’ $p$ ’ alike objects of one type ’ $q$ ’ alike objects of second type and ’ $r$ ’ alike of third type is

$ (p+1)(q+1)(r+1)-1 $

• Number of ways in which atleast one object may be selected from ’ $n$ ’ objects where ’ $p$ ’ alike of one type ’ $q$ ’ alike of second type and ’ $r$ ’ alike of third type and rest $n-(p+q+r)$ are different is

$(p+1)(q+1)(r+1) 2^{n-(p+q+r)}-1 $

Negative Binomial Expansion :

$\quad$ Coefficient of $X^{r}$ in expansion of

$(1-x)^{-n}={ }^{n+r-1} C_{r}(n \in N)$

Number of divisors :

$\quad$ Let $N=p^{a} q^{b} r^{c} \ldots .$. where $p, q, r \ldots \ldots$ are distinct primes & $a, b, c \ldots .$. are natural numbers then :

• The total numbers of divisors of $\mathrm{N}$ including 1 & N is $=(\mathrm{a}+1)(\mathrm{b}+1)(\mathrm{c}+1) \ldots \ldots \ldots$

• The sum of these divisors is $=$

$ \left(p^{0}+p^{1}+p^{2}+\ldots .+p^{a}\right)\left(q^{0}+q^{1}+q^{2}+\ldots .+q^{b}\right) \left(r^{0}+r^{1}+r^{2}+\ldots .+r^{c}\right) \ldots \ldots $

• Number of ways in which $\mathrm{N}$ can be resolved as a product of two factors is

  • $ =\frac{1}{2}(a+1)(b+1)(c+1) \ldots . \hspace{2mm} \text { if N is not a perfect square.} $

  • $ =\frac{1}{2}[(a+1)(b+1)(c+1) \ldots+1] \hspace{2mm} \text { if N is a perfect square.}$

• Number of ways in which a composite number $\mathrm{N}$ can be resolved into two factors which are relatively prime (or coprime) to each other is equal to $2^{n-1}$ where $\mathrm{n}$ is the number of different prime factors in $\mathrm{N}$.

Dearrangement :

$\quad$ Number of ways in which ’ $n$ ’ letters can be put in ’ $n$ ’ corresponding envelopes such that no letter goes to correct envelope is $n !\left(1-\frac{1}{1 !}+\frac{1}{2 !}-\frac{1}{3 !}+\frac{1}{4 !} \ldots \ldots \ldots .+(-1)^{n} \frac{1}{n !}\right)$

Principle of counting :

• The rule of sum: If a first task can be performed in m ways, while a second task can be performed in n ways, and the tasks cannot be performed simultaneously, then performing either one of these tasks can be accomplished in any one of total m+n ways.

• The rule of product: If a procedure can be broken down into first and second stages, and if there are m possible outcomes for the first stage and if, for each of these outcomes, there are n possible outcomes for the second stage, then the total procedure can be carried out, in the designated order, in a total of mn ways.

Factorial notation:

$n! = n(n-1)…(3)(2)(1)$

Permutation:

, The number of permutations of n different objects taken r at a time is represented as $ ^n P_r = \frac{n!}{(n-r)!}$

• Permutation with repetition: The no. of permutations of n different objects taken r at a time when each object may be repeated any number of times is $n^r$

• Permutation of alike objects: The number of permutations of n objects taken all at a time in which, p are alike objects of one kind, q are alike objects of second kind & r are alike objects of a third kind and the rest (n - (p + q + r)) are all different is $\frac{n!}{p! q! r!}$

• Permutation with Restriction: The number of permutations of n different objects, taken all at a time, when m specified objects always come together is $m! × (n - m + 1)!$

• Non-consecutive selection: $\quad \quad$• The number of selections of r consecutive objects out of n objects in a row $= n - r + 1$

$\quad \quad$• The number of selections of r consecutive objects out of n objects along a circle $= n , \text{~ when~} r < n$ $= 1 , \text{~ when~} r=n$

Circular permutation:

The number of circular arrangements of n different objects $= (n - 1)!$

• Clockwise and anticlockwise arrangements:

  • When clockwise and anticlockwise arrangements are not different, i.e. when observations can be made from both sides, the number of circular arrangements of n different objects is $\frac{(n - 1)!}{2}$

  • The number of circular permutation of n different objects taken all at a time is (n - 1)!, if clockwise and anticlockwise orders are taken as different.

  • The number of circular permutations of n different objects taken r at a time

    • $\frac{^nP_r}{r}$, when clockwise and anticlockwise orders are treated as different.

    • $\frac{^nP_r}{2r}$, when clockwise and anticlockwise orders are treated as same.

Properties of permutation:

• ${ }^n P_n=n(n-1)(n-2) \ldots 3 \times 2 \times 1=n$!

• ${ }^n P_0=\frac{n !}{n !}=1$

• ${ }^n P_1=n$

• ${ }^n P_{n-1}=n$ !

• ${ }^n P_r=n \cdot{ }^{n-1} P_{r-1}=n(n-1)^{n-2} P_{r-2}$

• ${ }^{n-1} P_r+r \cdot{ }^{n-1} P_{r-1}={ }^n P_r$

• $\frac{{ }^n P_r}{{ }^n P_{r-1}}=n-r+1$

Combination:

$\quad$ The number of all combinations of n objects, taken r at a time is generally denoted by C(n, r) or $^nC_r$ is $ ^n C_r = \frac{n!}{r! (n-r)!}$

• Properties:

  • ${ }^n C_r={ }^n C_{n-r}$

  • ${ }^n C_r+{ }^n C_{r-1}={ }^{n+1} C_r$

  • ${ }^n C_x={ }^n C_y \Rightarrow x=y$ or $x+y=n$

  • If $n$ is even, then the greatest value of ${ }^n C_r$ is ${ }^n C_{n / 2}$

  • If $n$ is odd, then the greatest value of ${ }^n C_r$ is ${ }^n C_{\frac{n+1}{2}}$

  • ${ }^n C_0+{ }^n C_r+$ .. $+{ }^n C_n=2^n$

  • ${ }^n C_n+{ }^{n+1} C_n+{ }^{n+2} C_n+$ $+{ }^{2 n-1} C_n={ }^{2 n} C_{n+1}$

Combinations under restrictions:

• Number of ways of choosing r objects out of n different objects if p particular objects must be excluded the required number of ways $= ^{n-p}C_r$

• Number of ways of choosing r objects out of n different objects if p particular objects must be included (p ≤ r). the required number of ways $= ^{n-p}C_{r-p}$

• The total number of combinations of n different objects taken one or more at a time $= 2^n - 1$

Combinations of alike objects:

$\quad$ If out of (p + q + r + s) objects, p are alike of one kind, q are alike of a second kind, r are alike of the third kind and s are different, then the total number of combinations is $(p + 1)(q + 1)(r + 1)2^s - 1$

Division into groups:

• The number of ways in which (m + n) different objects can be divided into two unequal groups containing m and n objects respectively is $\frac{(m+n)!}{m! n!}$

• If m = n, the groups are equal and in this case the number of division is $\frac{(2n)!}{n!n!2!} $; as it is possible to interchange the two groups without obtaining a new distribution.

• The number of ways in which mn different objects can be divided equally into m groups if order of groups is not important is $\frac{(m n)!}{(n!)^m m!}$

• The number of ways in which mn different objects can be divided equally into m groups if the order of groups is important is $\frac{(m n)!}{(n!)^m }$