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Combinatorics Difficulty 7.8 National olympiad, round 2 Find the answer

For every positive integer nn, denote by DnD_{n} the number of permutations \left(x_{1}, \ldots, x_{n}\right)of of (1,2, \ldots, n)suchthat such that x_{j} \neq jforevery for every 1 \leq j \leq n.For. For 1 \leq k \leq \frac{n}{2},denoteby, denote by \Delta(n, k)thenumberofpermutations(x1,,xn) the number of permutations \left(x_{1}, \ldots, x_{n}\right) of (1,2,,n)(1,2, \ldots, n) such that xi=k+ix_{i}=k+i for every 1ik1 \leq i \leq k and xjjx_{j} \neq j for every 1jn1 \leq j \leq n. Prove that Δ(n,k)=i=0k1(k1i)D(n+1)(k+i)n(k+i)\Delta(n, k)=\sum_{i=0}^{k-1}\binom{k-1}{i} \frac{D_{(n+1)-(k+i)}}{n-(k+i)}

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let ar{i1,,ik}{a1,,ak}a_{r} \in\left\{i_{1}, \ldots, i_{k}\right\} \cap\left\{a_{1}, \ldots, a_{k}\right\}. Thus ar=isa_{r}=i_{s} for some srs \neq r. Now there are two cases: Case 1. as{i1,,ik}a_{s} \in\left\{i_{1}, \ldots, i_{k}\right\}. Let as=ita_{s}=i_{t}. In this case a derangement x=(x1,,xn)x=\left(x_{1}, \ldots, x_{n}\right) satisfies the condition xij=ajx_{i_{j}}=a_{j} if and only if the derangement x=(x1,,xit1,xit+1,xn)x^{\prime}=\left(x_{1}^{\prime}, \ldots, x_{i_{t}-1}^{\prime}, x_{i_{t}+1}^{\prime}, x_{n}^{\prime}\right) of the set [n]\{it}[n] \backslash\left\{i_{t}\right\} satisfies the condition xij=ajx_{i_{j}}^{\prime}=a_{j}^{\prime} for all jtj \neq t, where aj=aja_{j}^{\prime}=a_{j} for jsj \neq s and as=ata_{s}^{\prime}=a_{t}. This provides a one to one correspondence between the derangements x=(x1,,xn)x=\left(x_{1}, \ldots, x_{n}\right) of [n][n] with xij=ajx_{i_{j}}=a_{j} for the given sets \left\{i_{1}, \ldots, i_{k}\right\}and{a1,,ak} and \left\{a_{1}, \ldots, a_{k}\right\} with \ell elements in their intersections, and the derangements x=(x1,,xit1,xit+1,xn)x^{\prime}=\left(x_{1}^{\prime}, \ldots, x_{i_{t}-1}^{\prime}, x_{i_{t}+1}^{\prime}, x_{n}^{\prime}\right) of [n]\{it}[n] \backslash\left\{i_{t}\right\} with xij=ajx_{i_{j}}=a_{j}^{\prime} for the given sets \left\{i_{1}, \ldots, i_{k}\right\} \backslash\left\{i_{t}\right\}and{a1,,ak}\{at} and \left\{a_{1}^{\prime}, \ldots, a_{k}^{\prime}\right\} \backslash\left\{a_{t}^{\prime}\right\} with 1\ell-1 elements in their intersections. Case 2. as{i1,,ik}a_{s} \notin\left\{i_{1}, \ldots, i_{k}\right\}. In this case a derangement x=(x1,,xn)x=\left(x_{1}, \ldots, x_{n}\right) satisfies the condition xij=ajx_{i_{j}}=a_{j} if and only if the derangement x=(x1,,xas1,xas+1,xn)x^{\prime}=\left(x_{1}^{\prime}, \ldots, x_{a_{s}-1}^{\prime}, x_{a_{s}+1}^{\prime}, x_{n}^{\prime}\right) of the set [n]\{as}[n] \backslash\left\{a_{s}\right\} satisfies the condition xij=ajx_{i_{j}}^{\prime}=a_{j} for all jsj \neq s. This provides a one to one correspondence between the derangements x=(x1,,xn)x=\left(x_{1}, \ldots, x_{n}\right) of [n][n] with xij=ajx_{i_{j}}=a_{j} for the given sets \left\{i_{1}, \ldots, i_{k}\right\}and{a1,,ak} and \left\{a_{1}, \ldots, a_{k}\right\} with \ell elements in their intersections, and the derangements x=(x1,,xas1,xas+1,xn)x^{\prime}=\left(x_{1}^{\prime}, \ldots, x_{a_{s}-1}^{\prime}, x_{a_{s}+1}^{\prime}, x_{n}^{\prime}\right) of [n]\{as}[n] \backslash\left\{a_{s}\right\} with xij=ajx_{i_{j}}=a_{j} for the given sets \left\{i_{1}, \ldots, i_{k}\right\} \backslash\left\{i_{s}\right\}and{a1,,ak}\{as} and \left\{a_{1}, \ldots, a_{k}\right\} \backslash\left\{a_{s}\right\} with 1\ell-1 elements in their intersections. These considerations show that Δ(n,k,)=Δ(n1,k1,1)\Delta(n, k, \ell)=\Delta(n-1, k-1, \ell-1). Iterating this argument we have Δ(n,k,)=Δ(n,k,0)\Delta(n, k, \ell)=\Delta(n-\ell, k-\ell, 0) We can therefore assume that =0\ell=0. We thus evaluate Δ(n,k,0)\Delta(n, k, 0), where 2kn2 k \leqslant n. For k=0k=0, we obviously have Δ(n,0,0)=Dn\Delta(n, 0,0)=D_{n}. For k1k \geqslant 1, we claim that Δ(n,k,0)=Δ(n1,k1,0)+Δ(n2,k1,0)\Delta(n, k, 0)=\Delta(n-1, k-1,0)+\Delta(n-2, k-1,0) For a derangement x=(x1,,xn)x=\left(x_{1}, \ldots, x_{n}\right) satisfying xij=ajx_{i_{j}}=a_{j} there are two cases: xa1=i1x_{a_{1}}=i_{1} or xa1i1x_{a_{1}} \neq i_{1}. If the first case occurs then we have to evaluate the number of derangements of the set [n]\{i1,a1}[n] \backslash\left\{i_{1}, a_{1}\right\} for the given sets \left\{i_{2}, \ldots, i_{k}\right\}and{a2,,ak} and \left\{a_{2}, \ldots, a_{k}\right\} with 0 elements in their intersections. The number is equal to Δ(n2,k1,0)\Delta(n-2, k-1,0). If the second case occurs then we have to evaluate the number of derangements of the set [n]\{a1}[n] \backslash\left\{a_{1}\right\} for the given sets \left\{i_{2}, \ldots, i_{k}\right\}and{a2,,ak} and \left\{a_{2}, \ldots, a_{k}\right\} with 0 elements in their intersections. The number is equal to Δ(n1,k1,0)\Delta(n-1, k-1,0). We now use induction on kk to show that Δ(n,k,0)=i=0k1(k1i)D(n+1)(k+i)n(k+i),22kn\Delta(n, k, 0)=\sum_{i=0}^{k-1}\binom{k-1}{i} \frac{D_{(n+1)-(k+i)}}{n-(k+i)}, \quad 2 \leqslant 2 k \leqslant n For k=1k=1 we have Δ(n,1,0)=Δ(n1,0,0)+Δ(n2,0,0)=Dn1+Dn2=Dnn1\Delta(n, 1,0)=\Delta(n-1,0,0)+\Delta(n-2,0,0)=D_{n-1}+D_{n-2}=\frac{D_{n}}{n-1} Now let the result be true for k1k-1. We can write Δ(n,k,0)=Δ(n1,k1,0)+Δ(n2,k1,0)=i=0k2(k2i)Dn(k1+i)(n1)(k1+i)+i=0k2(k2i)D(n1)(k1+i)(n2)(k1+i)=i=0k2(k2i)D(n+1)(k+i)n(k+i)+i=1k1(k2i1)Dn(k+i1)(n1)(k+i1)=D(n+1)knk+i=1k2(k2i)D(n+1)(k+i)n(k+i)+D(n+1)(2k1)n(2k1)+i=1k2(k2i1)D(n+1)(k+i)n(k+i)=D(n+1)knk+i=1k2[(k2i)+(k2i1)]D(n+1)(k+i)n(k+i)+D(n+1)(2k1)n(2k1)=D(n+1)knk+i=1k2(k1i)D(n+1)(k+i)n(k+i)+D(n+1)(2k1)n(2k1)=i=0k1(k1i)D(n+1)(k+i)n(k+i).\begin{aligned} \Delta(n, k, 0)= & \Delta(n-1, k-1,0)+\Delta(n-2, k-1,0) \\ = & \sum_{i=0}^{k-2}\binom{k-2}{i} \frac{D_{n-(k-1+i)}}{(n-1)-(k-1+i)}+\sum_{i=0}^{k-2}\binom{k-2}{i} \frac{D_{(n-1)-(k-1+i)}}{(n-2)-(k-1+i)} \\ = & \sum_{i=0}^{k-2}\binom{k-2}{i} \frac{D_{(n+1)-(k+i)}}{n-(k+i)}+\sum_{i=1}^{k-1}\binom{k-2}{i-1} \frac{D_{n-(k+i-1)}}{(n-1)-(k+i-1)} \\ = & \frac{D_{(n+1)-k}}{n-k}+\sum_{i=1}^{k-2}\binom{k-2}{i} \frac{D_{(n+1)-(k+i)}}{n-(k+i)} \\ & +\frac{D_{(n+1)-(2 k-1)}}{n-(2 k-1)}+\sum_{i=1}^{k-2}\binom{k-2}{i-1} \frac{D_{(n+1)-(k+i)}}{n-(k+i)} \\ = & \frac{D_{(n+1)-k}}{n-k}+\sum_{i=1}^{k-2}\left[\binom{k-2}{i}+\binom{k-2}{i-1}\right] \frac{D_{(n+1)-(k+i)}}{n-(k+i)}+\frac{D_{(n+1)-(2 k-1)}}{n-(2 k-1)} \\ = & \frac{D_{(n+1)-k}}{n-k}+\sum_{i=1}^{k-2}\binom{k-1}{i} \frac{D_{(n+1)-(k+i)}}{n-(k+i)}+\frac{D_{(n+1)-(2 k-1)}}{n-(2 k-1)} \\ = & \sum_{i=0}^{k-1}\binom{k-1}{i} \frac{D_{(n+1)-(k+i)}}{n-(k+i)} . \end{aligned}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.