$$\large2017^{(2020^{2015})} \pmod{13}$$

I am practicing for my exam and I can solve almost all problem, but this type of problem is very hard to me. In this case, I have to compute this by modulo $13$.

  • $\begingroup$ Well you can replace 2017 with 2, did you get that far? $\endgroup$ – Gregory Grant Jan 10 '16 at 14:08
  • $\begingroup$ I did just 2017^2020 mod 13 $\endgroup$ – josf Jan 10 '16 at 14:09
  • $\begingroup$ First off, is this $(2017)^{(2020^{2015})}$? Or is it $(2017^{2020})^{2015}$? $\endgroup$ – Gregory Grant Jan 10 '16 at 14:16
  • $\begingroup$ It is fix now.. $\endgroup$ – josf Jan 10 '16 at 14:23

As $2017\equiv2\pmod{13},$


Now as $(2020,\phi(13))=4,$ let us find $2020^{2015-1}\pmod{\dfrac{12}4}$

As $2020\equiv1\pmod3\implies2020^{2015-1}\equiv1\pmod3$




Let's try to see what we can infer from $2017\bmod{13}$:




This means that we can find the answer by representing $2020^{2015}$ as $\color\orange{12}m+n$:


Let's show by induction that $2020^k\equiv\color\purple{4}\pmod{\color\orange{12}}$:

  • Base case: $2020^1\equiv\color\purple{4}\pmod{\color\orange{12}}$
  • Assumption: $2020^k\equiv\color\purple{4}\pmod{\color\orange{12}}$
  • Inductive step: $2020^{k+1}\equiv2020^k\cdot2020^1\equiv\color\purple{4}\cdot\color\purple{4}\equiv\color\purple{4}\pmod{\color\orange{12}}$

Therefore $2020^{2015}=\color\orange{12}m+\color\purple{4}$, and this gives the final answer:



${\rm mod}\ \color{#c00}{12}\!:\, (\overbrace{4\!+\!12k}^{\large \rm e.g.\ 2020})^{\large 1+n}\!\equiv 4^{\large 1+n}\!\equiv\! \overbrace{4(4^{\large n}\!\bmod 3)}^{\rm\large mod\ Distribute}\!\equiv 4(1^{\large n}\!\bmod 3)\equiv \color{#c00}4,\ $ therefore

${\rm mod}\ 13\!:\,\ a^{\large (4+12k)^{\LARGE 1+n}}\!\equiv a^{\large\color{#c00}{4+12k}}\equiv \underbrace{a^{\large 4}(\color{#0a0}{a^{\large 12}})^{\large k}\!\equiv a^{\large 4} \color{#0a0}{\bf 1}^{\large k}\!}_{\large \rm by\ \color{#0a0}{\rm Fermat}\ \&\ 13\ \nmid\ a}\equiv a^{\large 4},\, $ so $\,{a\equiv 2017\equiv 2}\Rightarrow\,a^{\large 4}\!\equiv 16$

  • $\begingroup$ In the first line we used $\ 4b\bmod 4\!\cdot\!3 =\, 4(b\bmod 3 ).\ $ See here on this mod Distributive Law $\endgroup$ – Bill Dubuque Jan 10 '17 at 1:43

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