Mathematics

Example: Solve the system of linear congruences $x \equiv 2 ~(mod ~3)$, $x \equiv 3 ~(mod ~5)$, $x \equiv 2 ~(mod ~7)$. Answer:   Let $ n = 3 \cdot 5 \cdot 7 = 105, ~N_{1} = \dfrac{n}{3} = \dfrac{105}{3} = 35, ~ N_2 = \dfrac{n}{5} = \dfrac{105}{5} = 21, ~ N_3 = \dfrac{n}{7} = \dfrac{105}{7} = 15$. Now, we find solution of linear congruences $35 x_{1} \equiv 1 ~(mod ~3), ~ 2 1 x_2 \equiv 1 ~(mod ~5),  15x_3 \equiv 1 ~(mod ~7)$ We will find solution of linear congruences $35 x_{1} \equiv 1 ~(mod ~3), ~ 2 1 x_2 \equiv 1 ~(mod ~5),  15x_3 \equiv 1 ~(mod ~7)$ by trial and error method.  Firstly we find solution of linear congruence $35 x_{1} \equiv 1 ~(mod ~3)$. We will consider simple values of $x_{1}$ as $1, ~-1,~ 2,~ -2,~ \cdots $ and check which value satisfy given linear congruence $35 x_{1} \equiv 1 ~(mod ~3)$. When we take $x_{1} = 1, ~-1,~ 2,~ -2,~3, ~ -3$, we observe that $x_{1} =1, ~-1,$ are not a solution of $35 x_{1} \equiv 1 ...

  Chinese Remainder Theorem . Let $n_{1},~ n_2,~ \cdots,~ n_r$ be positive integers such that $gcd(n_i, n_j) = 1$  for $i \not = j$. Then the system of linear congruences $x \equiv a_{1} ~(mod ~n_{1})$ $x \equiv a_2 ~(mod~ n_2)$ $x \equiv a_3~ (mod~ n_3)$ $\vdots$ $x \equiv a_r~ (mod ~n_r)$ has a simultaneous solution, which is unique modulo the integer  $n_{1} \cdot n_2 \cdot n_3 ~\cdots~ n_r$. P roof: Consider $n = n_{1} \cdot n_2 \cdot n_3 ~\cdots~ n_r$. Let $N_k = \dfrac{n}{n_k} = n_{1} \cdot n_2 \cdot n_3 ~\cdots~ n_{k-1} \cdot n_{k+1} ~\cdots ~n_r $ for each $k = 1, ~2, ~3,~ \cdots, ~r $ i.e. $N_k$ is the product of all the integers $n_i$; with the factor $n_k$ omitted. Since $gcd(n_i, n_j) = 1$  for $i \not = j$, we will have $gcd(N_k, n_k) = 1$. By Theorem , the  linear congruence $N_k ~x \equiv 1 ~(mod ~n_k)$, has a solution; call the unique solution $x_k$. We claim that the integer  $\bar{x} = a_{1} \cdot N_...

Example: Solve the linear congruence $18x \equiv 30 ~(mod ~42)$.   Answer: Since $gcd(18, 42) = 6$ and $6$ divides $30$, by  Theorem , given linear congruence $18x \equiv 30 ~(mod ~42)$ has $6$ mutually incongruent solutions modulo $42$.  We will find first solution by trial and error method. We will consider simple values of $x$ as $1, ~-1,~ 2,~ -2,~ \cdots $ and check which value satisfy given linear congruence $18x \equiv 30 ~(mod ~42)$. When we take $x = 1, ~-1,~ 2,~ -2,~3, ~ -3$, we observe that $x =1, ~-1,~ 2,~ -2,~3$ are not a solution of $18x \equiv 30 ~(mod ~42)$. Now, we take $x = -3$, we observe that $x = -3$ is  a solution of $18x \equiv 30 ~(mod ~42)$. But $-3 \equiv 39 ~ (mod~ 42)$, So $x_{0} = 39$ is a solution of a given  linear congruence $18x \equiv 30 ~(mod ~42)$.  By Theorem , other solutions of given linear congruence $18x \equiv 30 ~(mod ~42)$  are given by  $x_{1} = 39 + \frac{42}{6} = 39 + 7 = 46 \equiv 4~ (mod ~ 42)$,...

 Example:  Solve the linear congruence $9x \equiv 21 ~(mod ~ 30)$. Answer: Since $gcd(9, 30) = 3$ and $3$ divides $21$, by  Theorem , given linear congruence $9x \equiv 21 ~(mod ~30)$ has $3$ mutually incongruent solutions modulo $30$.  We will find first solution by trial and error method. We will consider simple values of $x$ as $1, ~-1,~ 2,~ -2,~ \cdots $ and check which value satisfy given linear congruence $9x \equiv 21 ~(mod~ 30)$. When we take $x = 1$, we observe that $x =1$ is not a solution of $9x \equiv 21 ~(mod ~30)$. Now, we take $x = - 1$, we observe that $x = - 1$ is  a solution of $9x \equiv 21 ~(mod ~30)$. But $-1 \equiv 29 ~(mod ~30) $, therefore $x_{0} = 29$ is a solution of a given  linear congruence $9x \equiv 21 ~(mod ~30)$.  By Theorem , other solutions of given linear congruence $9x \equiv 21 ~(mod ~30)$  are given by $x_{1} = 29 + \frac{30}{3} = 29 + 10 = 39 \equiv 9~ (mod ~ 30), ~ x_2 = 29 + (\frac{30}{3}) ~2 = 29 + 10 \...

  Lemma: Let $p$ be a prime number. Then prove that $x^2 \equiv 1 ~(mod ~p)$ if and only if $x \equiv 1 ~ (mod ~p) $ or $x \equiv - 1 ~ (mod ~p) $. P roof: Suppose that $x^2 \equiv 1 ~(mod ~p)$. This quadratic congruence is equivalent to $(x^2 -1 )\equiv 0 ~(mod ~p)$ i.e.  $(x -1 ) ~(x + 1) \equiv 0 ~(mod ~p)$. This implies that $p ~|~ (x - 1) ~ (x+1)$. By Theorem , $p ~|~ (x - 1)$ or $p ~|~ (x + 1)$. This gives that $x \equiv 1 ~ (mod ~p) $ or $x \equiv - 1 ~ (mod ~p) $. Conversely, suppose that either $x \equiv 1 ~ (mod ~p) $ or $x \equiv - 1 ~ (mod ~p) $. Then clearly $x^2 \equiv 1 ~(mod ~p)$.

  Definition:   Let $a, ~b,  ~n > 0$ be integers and $x$ be a variable then  an equation of the form $ax \equiv b ~(mod ~n)$  is called a linear congruence, and an integer $x_{0}$ for which $a x_{0} \equiv b ~(mod ~n )$ is called a solution of an equation  $ax \equiv b ~(mod ~n)$.  By definition, $a ~x_{0} \equiv b ~(mod~ n)$ if and only  if $n ~|~ a x_{0} - b $ i.e. $a x_{0} - b = n y_{0} $ for some integer $y_{0}$· Thus, the problem of finding all integers that will satisfy the linear congruence $a x \equiv b ~(mod ~n)$ is identical with that of obtaining all solutions of the linear Diophantine equation $ax - ny = b$. Theorem:   The linear congruence $ax \equiv b ~(mod ~n)$ has a solution if and only if $d ~|~ b$, where $d = gcd(a, n)$. If $d ~|~ b$, then it has $d$ mutually incongruent solutions modulo $n$. P roof: In last para, we have observed that the given linear congruence $ax \equiv b ~(mod ~n)$ is equivalent to the linea...

  Example : Show that the integer $111^{333} + 333^{111}$ is divisible by $7$. Answer: Firstly, we will find the remainder when $111^{333} $ and $ 333^{111}$ is divided by $7$. Then with help of property $4.$ of Theorem , we show that  the integer $111^{333} + 333^{111}$ is divisible by $39$. We have $111 \equiv 6 ~(mod ~7)$ but $6 \equiv -1 ~(mod ~7) $. Therefore $111 \equiv -1 ~(mod ~7) $. By property $6.$ of Theorem , we have  $111^{2} \equiv 1 ~(mod ~7)$. We can write $333$ as $333 = 166 \cdot 2 + 1$. Therefore $111^{333} \equiv (111^{2})^{166} \cdot 111 \equiv 1^{166} \cdot 111 \equiv  6 ~(mod ~7)$, i.e. $111^{333} \equiv  6 ~(mod ~7)$. This gives that $6$ is a remainder when $111^{333}$ is divided by $7$. We have $333 \equiv 4 ~(mod ~7)$.  By property $6.$ of Theorem , we have $333^2 \equiv 4^2 \equiv 2 ~(mod ~7)$. Again, with the help of property $6.$ of Theorem , we have  $(333^2)^3 = 333^6 \equiv 2^3 \equiv 1 ~(mod ~7)$. We ca...

Example : Show that the integer $53^{103} + 103^{53}$ is divisible by $39$.  Answer: Firstly, we will find the remainder when $53^{103} $ and $ 103^{53}$ is divided by $39$. Then with help of property $4.$ of Theorem , we show that  the integer $53^{103} + 103^{53}$ is divisible by $39$. We have $53 \equiv 14 ~(mod ~39)$. By property $6.$ of Theorem , we have $53^2 \equiv 14^2 \equiv 1 ~(mod ~39)$. We can write $103$ as $103 = 51 \cdot 2 + 1$. Therefore $53^{103} \equiv (53^{2})^{51} \cdot 53 \equiv 1^{51} \cdot 53 \equiv  14 ~(mod ~39)$, i.e. $53^{103} \equiv  14 ~(mod ~39)$. This gives that $14$ is a remainder when $53^{103}$ is divided by $39$. We have $103 \equiv 25 ~(mod ~39)$. But $25 \equiv -14 ~ (mod ~ 39)$. Therefore $103 \equiv -14 ~(mod ~39)$. By property $6.$ of Theorem , we have $103^2 \equiv (- 14)^2 \equiv 1 ~(mod ~39)$.  We can write $53$ as $53 = 26 \cdot 2 + 1$. Therefore $103^{53} \equiv (103^{2})^{26} \cdot 103 \equiv 1^{26} \...

  Example: Find a remainder when $2^{50}$ is divided by $7$.  Answer: We have $2 \equiv 2 ~(mod ~7) $. By property $6.$ of Theorem ,  we have  $2^{2} \equiv 4 ~(mod ~7)$. On the same line we have $2^{3} \equiv 1 ~(mod ~7)$. We can write $50$ as $50 = 16 \cdot 3 + 2$. Therefore $2^{50} \equiv (2^{3})^{16} \cdot 2^2 \equiv 1^{48} \cdot 2^2 \equiv  4 ~(mod ~7)$, i.e. $2^{50} \equiv  4 ~(mod ~7)$. This gives that $4$ is a remainder when $2^{50}$ is divided by $7$. Example:   Find a remainder when $41^{65}$ is divided by $7$.   Answer: We have $41 \equiv 6 ~(mod ~7) $, but $6 \equiv -1 ~(mod ~7) $. Therefore $41 \equiv -1 ~(mod ~7) $. By property $6.$ of Theorem ,  we have  $41^{2} \equiv 1 ~(mod ~7)$.  We can write $65$ as $65 = 32 \cdot 2 + 1$. Therefore $41^{65} \equiv (41^{2})^{32} \cdot 41 \equiv 1^{32} \cdot 41 \equiv  6 ~(mod ~7)$, i.e. $41^{65} \equiv  6 ~(mod ~7)$. This gives that $6$ is a remainder whe...

 In Theorem we saw that if $a \equiv b ~(mod ~n)$, then $c \cdot a \equiv c \cdot b ~(mod ~n)$ for any integer $c$. What about converse $?$ i.e. if $c \cdot a \equiv c \cdot b ~(mod ~n)$ for any integer $c$ does it implies  $a \equiv b ~(mod ~n) ~ ?$  we give answer to this question as No. Here is an example $2 \cdot 4  \equiv 2 \cdot 1 ~(mod~6)$, but $4 \not\equiv 1 ~(mod ~6)$. In brief: One cannot unrestrictedly cancel a common factor in the arithmetic of congruences.  Theorem:   If $c \cdot a \equiv c \cdot b ~(mod ~n)$, then $a \equiv b ~(mod ~\frac{n}{d} )$, where $d = gcd(c, n)$.  Proof:   Suppose that $c \cdot a \equiv c \cdot b ~(mod ~n)$. Therefore by definition of congruences $c \cdot a ~-~ c \cdot b = n \cdot k$ for some integer $k$. Since  $gcd(c, n) = d$, there exist relatively prime integers $r$ and $s$ such that  $c = d \cdot r,~ n = d \cdot s$. We put $c = d \cdot r,~ n = d \cdot s$ in $c \cdot a - c \cdot b = n \cdo...