In an attempt to prove a statement about the residues of a certain sequence mod ##10^n##, I've derived something which seems to be in direct violation of Carmichael's theorem. Of course, this can't be right, so can someone either explain what bit of my reasoning is wrong or why this isn't in violation of Carmichael's Theorem? First of all, let ##\lambda## be the Carmichael function, and let ##k## be coprime to 2 and 5.(adsbygoogle = window.adsbygoogle || []).push({});

First of all, notice that, by Euler's theorem, ##k^{4\cdot 5^{n-1}}\equiv1\pmod{5^n}## and ##k^{2^{n-1}}\equiv1\pmod{2^n}##. This makes it clear by induction that ##a\equiv b\pmod{4\cdot 5^{n-1}}\rightarrow k^a\equiv k^b\pmod{5^n}## and ##a\equiv b\pmod{2^{n-1}}\rightarrow k^a\equiv k^b\pmod{2^n}##.

Let ##n\ge2## and ##a\equiv b\pmod{10^n}##. Then, as ##\left.2^{n-1},4\cdot 5^{n-1}\right|10^n##, ##a\equiv b\pmod2^{n-1}## and ##a\equiv b\pmod5^{n-1}##, so ##k^a\equiv k^b\pmod{2^n}## and ##k^a\equiv k^b\pmod{5^n}##. Therefore ##k^a\equiv k^b\pmod{\mathrm{lcm}\left(2^n,5^n\right)}##, so ##k^a\equiv k^b\pmod{10^n}##.

Letting ##a=10^n## and ##b=0##, we get ##k^{10^n}\equiv k^0=1\pmod{10^n}##.

As this holds for all ##k## coprime to ##10^n##, this means ##\left.\lambda\left(10^n\right)\right|10^n##. (This should be obvious enough; I should be able to provide a proof if necessary.) However, as ##10^n## is not a power of 2, Carmichael's theorem tells us that ##\lambda\left(10^n\right)=\varphi\left(10^n\right)=4\cdot 10^{n-1}##, which doesn't divide ##10^n##.

Anyone know what's wrong here?

**Physics Forums - The Fusion of Science and Community**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# This appears to be in direct violation of Carmichael's theorem.

Loading...

Similar Threads - appears direct violation | Date |
---|---|

I Demo of cosine direction with curvilinear coordinates | Dec 6, 2017 |

I Proving a proposition directly | Sep 13, 2016 |

Direct product representation of a function? | Jun 18, 2015 |

Why cross product has a direction but dot product doesn't? | Apr 17, 2015 |

**Physics Forums - The Fusion of Science and Community**