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Number theory Difficulty 6.2 National olympiad Prove it Saudi Arabia

For any positive integer nn, show that there exists a positive integer mm such that nn divides 2016m+m2016^{m} + m.

Solution

We generalize the problem to the following problem:
Let aa be a given positive integer. For every natural nn, there is a positive integer mm such that nn divides am+ma^{m} + m.

In fact, we proceed by induction on nn. Obviously this statement holds for n=1n = 1. Now assume n>1n > 1 and this statement holds for every natural number less than nn. Consider two cases:

Case 1: n=pn = p is a prime. If pap \mid a we are done. If not, take m=(a+1)(p1)+1m = (a+1)(p-1) + 1. Then
am+m=a(a+1)(p1)+1+(a+1)(p1)+1(a+1)p0(modp). a^{m} + m = a^{(a+1)(p-1)+1} + (a+1)(p-1) + 1 \equiv (a+1)p \equiv 0 \pmod{p}.

Case 2: nn is a composite. Let pp be the largest prime divisor of nn. Write n=pbq1a1qkakn = p^{b} q_{1}^{a_{1}} \cdots q_{k}^{a_{k}} as the prime factorization of nn and put n=pn1n = p n_{1}.

By induction hypothesis (to n1<nn_{1} < n), there is a positive integer kk such that ak+k0(modn1)a^{k} + k \equiv 0 \pmod{n_{1}}. This leads to ak+k=n1qa^{k} + k = n_{1} q, and we represent q=pq1+rq = p q_{1} + r with 0r<p0 \leqslant r < p. Thus, ak+k=n1(pq1+r)rn1(modn)a^{k} + k = n_{1}(p q_{1} + r) \equiv r n_{1} \pmod{n}. Now, put A=(q11)(qk1)A = (q_{1} - 1) \cdots (q_{k} - 1), or 11 if nn has only one prime divisor.

Since pp is the largest prime divisor of nn, it follows that AA and pp are coprime. Hence, there is a positive integer cc such that
Ac1(modp)(p1)rAc(p1)rr(modp). A c \equiv 1 \pmod{p} \Longleftrightarrow (p-1) r A c \equiv (p-1) r \equiv -r \pmod{p}.
This leads to
pb1q1a1qkakr(p1)Acpb1q1a1qkakr(modpbq1a1qkak). p^{b-1} q_{1}^{a_{1}} \cdots q_{k}^{a_{k}} r (p-1) A c \equiv -p^{b-1} q_{1}^{a_{1}} \cdots q_{k}^{a_{k}} r \pmod{p^{b} q_{1}^{a_{1}} \cdots q_{k}^{a_{k}}}.
Hence, we have
q1qkφ(n)rcn1r(modn). q_{1} \cdots q_{k} \varphi(n) r c \equiv -n_{1} r \pmod{n}.
Finally, define d=q1qkcd = q_{1} \cdots q_{k} c or cc, if nn has one prime divisor. We get rφ(n)dn1r(modn)r \varphi(n) d \equiv -n_{1} r \pmod{n}. Put m=rφ(n)d+km = r \varphi(n) d + k. Then,
am+marφ(n)d+k+rφ(n)d+kak+kn1rn1r+rφ(n)dn1rn1r0(modn) \begin{aligned} a^{m} + m &\equiv a^{r \varphi(n) d + k} + r \varphi(n) d + k \equiv a^{k} + k - n_{1} r \\ &\equiv n_{1} r + r \varphi(n) d \equiv n_{1} r - n_{1} r \equiv 0 \pmod{n} \end{aligned}
\square

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