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Number theory Difficulty 6.3 National olympiad Prove it

Example 2 Let a,mNa, m \in \mathbf{N}^{*}. Prove:
amamφ(m)(modm).a^{m} \equiv a^{m-\varphi(m)}(\bmod m) .

Solution

Notice, when (a,m)=1(a, m)=1, this conclusion is precisely Euler's theorem, hence, this conclusion can be regarded as a generalization of Euler's theorem.

If one of a,ma, m equals 1, the proposition is obvious, so we assume a,ma, m are both greater than 1.

Let m=m1m2m=m_{1} \cdot m_{2}, where the prime factors of m1m_{1} are all prime factors of aa, and (a,m2)=1\left(a, m_{2}\right)=1. Consequently, (m1,m2)=1\left(m_{1}, m_{2}\right)=1. To prove (2) holds, we only need to prove respectively:
amamq(m)(modm2)amamq(m)(modm1)\begin{array}{l} a^{m} \equiv a^{m-q(m)}\left(\bmod m_{2}\right) \\ a^{m} \equiv a^{m-q(m)}\left(\bmod m_{1}\right) \end{array}

Since (3) aφ(m)1(modm2)\Leftrightarrow a^{\varphi(m)} \equiv 1\left(\bmod m_{2}\right) (because (a,m2)=1\left(a, m_{2}\right)=1), and φ(m)=φ(m1\varphi(m)=\varphi\left(m_{1}\right., m2)=φ(m1)φ(m2)\left.m_{2}\right)=\varphi\left(m_{1}\right) \varphi\left(m_{2}\right) (the proof of this conclusion is given in the exercises). Using Euler's theorem, we know aq(m2)1(modm2)a^{q\left(m_{2}\right)} \equiv 1\left(\bmod m_{2}\right), hence (aqϕm2))q(m1)1(modm2)\left(a^{\left.q \phi m_{2}\right)}\right)^{q\left(m_{1}\right)} \equiv 1\left(\bmod m_{2}\right), i.e.,
aϕ(m)1(modm2),a^{\phi(m)} \equiv 1\left(\bmod m_{2}\right),

so, (3) holds.
For (4), we only need to prove: m1amα(m)m_{1} \mid a^{m-\alpha(m)}. For this, we only need to prove: for any prime factor pp of m1m_{1}, we have
vp(m1)vp(a)(mφ(m))v_{p}\left(m_{1}\right) \leqslant v_{p}(a)(m-\varphi(m))

In fact, by the definition of m1m_{1}, we know vp(m1)=vp(m)1,vp(a)1v_{p}\left(m_{1}\right)=v_{p}(m) \geqslant 1, v_{p}(a) \geqslant 1, thus, we have
vp(m1)=vp(m)2vp(m)1pρvρ(m)1(p1)pvρ(m)1(p1)pvρ(m)1φ(mpvρ(m))=pvρ(m)φ(mpvρ(m))pvρ(m)1φ(mpvρ(m))pvρ(m)φ(mpρvρ(m))φ(pvρ(m))φ(mpvρ(m))=pvρ(m)φ(mpρvρ(m))φ(m)pvρ(m)mpρvρ(m)φ(m)=mφφ(m)vp(a)(mφ(m))\begin{aligned} v_{p}\left(m_{1}\right) & =v_{p}(m) \leqslant 2^{v_{p}(m)-1} \leqslant p_{\rho}^{v_{\rho}(m)-1} \\ & \leqslant(p-1) p^{v_{\rho}(m)-1} \\ & \leqslant(p-1) p^{v_{\rho}(m)-1} \varphi\left(\frac{m}{p^{v_{\rho}(m)}}\right) \\ & =p^{v_{\rho}(m)} \varphi\left(\frac{m}{p^{v_{\rho}(m)}}\right)-p^{v_{\rho}(m)-1} \varphi\left(\frac{m}{p^{v_{\rho}(m)}}\right) \\ & \leqslant p^{v_{\rho}(m)} \varphi\left(\frac{m}{p_{\rho}^{v_{\rho}(m)}}\right)-\varphi\left(p^{v_{\rho}(m)}\right) \varphi\left(\frac{m}{p^{v_{\rho}(m)}}\right) \\ & =p^{v_{\rho}(m)} \varphi\left(\frac{m}{p_{\rho}^{v_{\rho}(m)}}\right)-\varphi(m) \\ & \leqslant p^{v_{\rho}(m)} \frac{m}{p_{\rho}^{v_{\rho}(m)}}-\varphi(m) \\ & =m^{-} \varphi \varphi(m) \\ & \leqslant v_{p}(a)(m-\varphi(m)) \end{aligned}

So, (5) holds, and consequently, (4) holds.
In summary, the proposition holds.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.