Example 4. Let p be a prime number. Can p divide 1994p−1994?
Solution
Given that 2(19942−1994),3(19943− 1994), we conjecture that p∣(1994p−1994).
To prove the above conjecture, we first prove a more general conclusion: If p is a prime, then p∣(np−n),(n∈N). (1) When n=1, it is obvious that p∣(1p−1). (2) Assume that when n=k, p∣(kp−k). Then when n=k+1, (k+1)p−(k+1)=∑r=0pCpr⋅kp−r−(k+1)=(kp−k)+∑r=1p−1Cprkp−r.∵rCpr=pCp−1r−1(1⩽r⩽p−1),Cpr,Cp−1r−1∈ N, and p is a prime, ∴p∣Cpr(1⩽r⩽p−1).
According to the inductive hypothesis, we know p∣[(k+1)p−{k+1}], i.e., n=:k+1. is true. Based on (i) and (2), we can conclude that for n∈N, p∣(np−n) always holds. Thus, p can divide 1994p−1994.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: NuminaMath-1.5,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.