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Number theory Difficulty 6.0 National olympiad Prove it Silk Road Mathematics Competition

Is there a positive integer sequence a1,a2,a_1, a_2, \dots that contains every positive integer exactly once and such that
τ(nan+1n+(n+1)ann+1) \tau(na_{n+1}^n + (n+1)a_n^{n+1})
is divisible by nn for every positive integer nn? (Here τ(n)\tau(n) is the number of positive divisors of nn.)

Solution

From the statement we have
τ(nan+1n+(n+1)ann+1)n.(1) \tau(na_{n+1}^n + (n+1)a_n^{n+1}) \vdash n. \qquad (1)
Let a1=1a_1 = 1 and a2=2a_2 = 2. Suppose that we have constructed the numbers a1,a2,,ama_1, a_2, \dots, a_m so that they are different natural numbers and (1) satisfied for each n=1,2,,m1n = 1, 2, \dots, m-1. Let LL be the smallest positive integer that does not occur among the numbers a1,a2,,ama_1, a_2, \dots, a_m. We set am+2=La_{m+2} = L and choose am+1a_{m+1} so that am+1>a1+a2++am+La_{m+1} > a_1 + a_2 + \dots + a_m + L and (1) is true for n=mn = m and n=m+1n = m+1. Repeating this process we obtain a permutation of all positive integers satisfying (1) for any positive integer nn.

For a prime pp and a positive integer kk, we denote by ψp(k)\psi_p(k) the largest integer ll such that kk is divisible by plp^l.

Lemma. Given positive integers a,b,k,l,Ma, b, k, l, M. Then there is a prime number p>Mp > M and a positive integer xx such that ψp(axk+b)=l\psi_p(ax^k + b) = l.

Proof. Let d=M+k+bd = M + k + b and pp be a prime divisor of abk1(d!)k+1ab^{k-1}(d!)^k + 1. Then p>d>Mp > d > M. By induction on NN, we prove that for any positive integer NN there exists a positive integer xx such that the number axk+bax^k + b is divisible by pNp^N.

Base: for N=1N = 1 the number x=d!bx = d!b.

Suppose that for NN there exist positive integers xx and tt such that axk+b=pNtax^k + b = p^N t. Let zz be a positive integer such that the number axn1zk+tax^{n-1} z k + t is divisible by pp (such a number exists, since the number pp is coprime with d!ad!a, and hence with the number akxakx). Then
a(x+zpN)k+baxk+axk1zpNk+bpNt+axk1zpNkpN(axk1zk+t)0(modpN+1), \begin{aligned} a(x + z p^N)^k + b &\equiv a x^k + a x^{k-1} z p^N k + b \equiv p^N t + a x^{k-1} z p^N k \\ &\equiv p^N(a x^{k-1} z k + t) \equiv 0 \pmod{p^{N+1}}, \end{aligned}
and the induction step is proved.

Therefore, there are positive integers x,tx, t with axk+b=pl+1ta x^k + b = p^{l+1} t. Then
a(x+pl)k+baxk+axk1pk+baxk1kpl(modpl+1), a(x + p^l)^k + b \equiv a x^k + a x^{k-1} p k + b \equiv a x^{k-1} k p^l \pmod{p^{l+1}},
i.e. ψp(a(x+pl)k+b)=l\psi_p(a(x + p^l)^k + b) = l, and the lemma is proved.

From the lemma for a=m,b=(m+1)amm+1,M=1,k=m,l=m1a = m, b = (m+1)a_m^{m+1}, M = 1, k = m, l = m-1 there is a prime p>1p > 1 and a positive integer xx such that ψp(mxm+(m+1)amm+1)=m1\psi_p(m x^m + (m+1)a_m^{m+1}) = m-1 and from the lemma for a=m+2,b=(m+1)km+1,M=p,k=m+2,l=ma = m+2, b = (m+1)k^{m+1}, M = p, k = m+2, l = m there is a prime q>pq > p and a positive integer yy such that ψq((m+1)km+1+(m+2)ym+2)=m\psi_q((m+1)k^{m+1} + (m+2)y^{m+2}) = m. By the Chinese remainder theorem, there is a positive integer zz such that z>a1+a2++am+L,zx(modpm)z > a_1 + a_2 + \dots + a_m + L, z \equiv x \pmod{p^m} and zy(modqm+1)z \equiv y \pmod{q^{m+1}}. Then it is easy to see that am+1=za_{m+1} = z works.

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