# Remainder when $\binom{2018}{1009}$ is divided by $2017^2?$

How can I find the remainder when $\displaystyle\binom{2018}{1009}$ is divided by $2017^2?$

My Approach:
I have used Lucas' theorem Extension here,that is $$\binom{n}{m} \equiv \frac{P}{Q}(\mod p^f)$$ $$\text{where},P=\prod_{i=0}^{s-f+1} \binom{n_i+n_{i+1}p+ \dots+n_{i+f-1}p^{f-1}}{m_i + m_{i+1}p+ \dots+ m_{i+f-1}p^{f-1}}$$ $$\& \space Q=\prod_{i=1}^{s-f+1} \binom{n_i+n_{i+1}p+ \dots+n_{i+f-2}p^{f-2}}{m_i + m_{i+1}p+ \dots+ m_{i+f-2}p^{f-2}}$$ so applying Lucas' theorem Extension in this problem: $$\text{We can write : } \binom{2018}{1009}= \binom{1 \times 2017 \quad \quad 1 \times 2017^0}{0 \times 2017 \quad \quad 1009 \times 2017^0}$$ $$\implies s=1,f=2$$ $$\therefore P= \prod_{i=0}^{0} \binom{n_i+n_{i+1}p}{m_i + m_{i+1}p} \space \& \space Q= \prod_{i=1}^{0} \binom{n_i}{m_i }$$ $$\implies P= \binom{1 \times 2017 \quad \quad 1 \times 2017^0}{0 \times 2017 \quad \quad 1009 \times 2017^0} \space \& \space Q= 1$$ so again we are getting: $$\binom{2018}{1009}=\frac{\binom{2018}{1009}}{1} \mod (2017^2)$$

Thus, ${\color{red}{\text{NO IMPROVEMENT}}}$
so how can i evaluate its remainder??
and if possible then how can we modify this process to get the answer? please help...

## 2 Answers

Note that

$$\binom{2018}{1009}=\frac{2018!}{1009!^2}=2018\cdot 2017\frac{2016!}{1009!^2}\equiv x \pmod{2017^2}\\\iff 2018\cdot \frac{2016!}{1009!^2}\equiv \frac{x}{2017} \pmod{2017}$$

and since

$$(p-1)!\equiv -1 \pmod{p}$$

$$\left[\left(\frac{p-1}{2}\right)!\right]^{2}\equiv (-1)^{\frac{p+1}{2}}\pmod{p}$$

we have that $$\frac{x}{2017}\equiv2018\frac{2016!}{1009!^2} \equiv \frac{2016!}{1009^2\cdot1008!^2}\equiv \frac{1}{1009^2} \iff \frac{x\cdot 1009^2}{2017}\equiv 1 \pmod{2017}$$

note that

$$1009^2=\left( \frac{2017+1}{2}\right)^2=\frac14(2017^2+2\cdot2017+1)\equiv\frac14 \pmod{2017}$$

thus

$$\frac{x\cdot 1009^2}{2017}\equiv 1 \iff \frac{x}{4\cdot 2017}\equiv 1 \pmod{2017}\iff x \equiv 8068 \pmod{2017^2}$$

• How ((2016!)/((1009^2)*((1008!)^2)) == 1/(1009^2) ? ,please explain – Suresh Jan 21 '18 at 3:57
• @user9198116 $2016!\equiv -1$ for wilson theorem and $(1008!)^2\equiv-1$ for math.stackexchange.com/questions/131175/… – gimusi Jan 21 '18 at 6:51

Hint: make use of the binomial identity $${2n\choose n} =2^n\frac{1\times 3\times 5\times\cdots\times (2n-1)}{n!}$$