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Number theory Difficulty 5.8 AIME, harder Prove it

Example 4. Let pp be a prime number. Can pp divide 1994p19941994^{p}-1994?

Solution

Given that 2(199421994),3(199432\left|\left(1994^{2}-1994\right), 3\right|\left(1994^{3}-\right. 1994), we conjecture that p(1994p1994)p \mid\left(1994^{p}-1994\right).

To prove the above conjecture, we first prove a more general conclusion: If pp is a prime, then p(npn),(nN)p \mid\left(n^{p}-n\right),(n \in N).
(1) When n=1n=1, it is obvious that p(1p1)p \mid\left(1^{p}-1\right).
(2) Assume that when n=kn=k, p(kpk)p \mid\left(k^{p}-k\right).
Then when n=k+1n=k+1,
(k+1)p(k+1)=r=0pCprkpr(k+1)=(kpk)+r=1p1Cprkpr.rCpr=pCp1r1(1rp1),Cpr,Cp1r1 \begin{array}{l} (k+1)^{p}-(k+1) \\ =\sum_{r=0}^{p} C_{p}^{r} \cdot k^{p-r}-(k+1) \\ =\left(k^{p}-k\right)+\sum_{r=1}^{p-1} C_{p}^{r} k^{p-r} . \\ \because r C_{p}^{r}=p C_{p-1}^{r-1}(1 \leqslant r \leqslant p-1), C_{p}^{r}, C_{p-1}^{r-1} \in \end{array}
NN, and pp is a prime,
pCpr(1rp1) \therefore p \mid C_{p}^{r}(1 \leqslant r \leqslant p-1) \text {. }

According to the inductive hypothesis, we know
p[(k+1)p{k+1}], i.e., n=:k+1 p \mid\left[(k+1)^{p}-\{k+1\}\right] \text {, i.e., } n=: k+1 \text {. }
is true.
Based on (i) and (2), we can conclude that for nNn \in N, p(npn)p \mid\left(n^{p}-n\right) always holds.
Thus, pp can divide 1994p19941994^{p}-1994.

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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.