Sunday 21 August 2016

number theory - Finding the last 4 digits of a huge power




I know this is more of a 'aops' type of question but here we go, I went to this math competition last year and there was this one problem that clearly I didn't solve but it recently came back to my mind and I want to know how to go about such problem:



Find the last 4 digits of the number: $$2^{{10}^{2018}}$$




My intuition is that one should probably use modular arithmetic on this one, the first things that came to my mind when I saw this one where: Chinese remainder Theorem and Binomial sums, I wasn't able to do much unfortunately...
I've read through the "How do I compute $a^b$ (mod c) by hand?" question but most of the answers rely on a and c being coprime which in my case $(2,10^4)=2$ is not true, the answers cover a few cases when a and c are not coprime but nothing very similar to my case...


Answer



You have $10000 = 2^45^4 = 16\cdot 625.$ You need to find $2^{10^{2018}} \pmod{10000}$ and the Chinese Remainder Theorem will do this nicely. First



$$2^{10^{2018}} \equiv 0 \pmod{16}.$$



Second, note that $\phi(625)=500$, so since $500$ divides $10^{2018},$



$$2^{10^{2018}} \equiv 1 \pmod{625}.$$




Then CRT gives $9376$ for the final answer.


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