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Number theory Difficulty 6.3 National Olympiad Prove it Austria

Determine the number of non-negative integers N<1000000=106N < 1000000 = 10^6 with the following property: There exists an integer exponent kk with 1k431 \le k \le 43 such that 20122012 is a divisor of Nk1N^k - 1.

Solution

It is obvious that NN and 20122012 must be relatively prime. If Nk1(modn)N^k \equiv 1 \pmod{n} and Nm1(modn)N^m \equiv 1 \pmod{n} both hold, so does Nd1(modn)N^d \equiv 1 \pmod{n} for d=gcd(k,m)d = \gcd(k, m). From m=φ(n)m = \varphi(n), we see that Nk1(modn)N^k \equiv 1 \pmod{n} implies that there exists a divisor dd of φ(n)\varphi(n) with Nd1(modn)N^d \equiv 1 \pmod{n}. Since 2012=45032012 = 4 \cdot 503 and φ(503)=502=2251\varphi(503) = 502 = 2 \cdot 251 (where 503503 and 251251 are both prime), the only possible exponents dd with Nd1(mod503)N^d \equiv 1 \pmod{503} of interest to us are 11, 22, 251251 and 502502. We need therefore only consider the exponents d=1d=1 and d=2d=2. Since N11N^1 \equiv 1 automatically implies N21N^2 \equiv 1 we only require the rests +1+1 and 1-1 modulo 503503. Since N21(mod4)N^2 \equiv 1 \pmod{4} holds for all odd values of NN, the values of nn with the required property are exactly the numbers N=1006u+1N = 1006 \cdot u + 1 and N=1006v1N = 1006 \cdot v - 1. Since 1006995=10009701006 \cdot 995 = 1000970 and 1006994=9999641006 \cdot 994 = 999964, only 0u9940 \le u \le 994 and 1v9941 \le v \le 994 are possible, and the required number of integers NN is equal to 995+994=1989995 + 994 = 1989.
\square

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