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Algebra Difficulty 8.3 Shortlist Find the answer

Does there exist a sequence (an)(a_{n}) of complex numbers such that for every positive integer pp we have that n=1anp\sum_{n=1}^{\infty} a_{n}^{p} converges if and only if pp is not a prime?

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Solution

The answer is YES. We prove a more general statement; suppose that N=CDN=C \cup D is an arbitrary decomposition of NN into two disjoint sets. Then there exists a sequence (an)n=1(a_{n})_{n=1}^{\infty} such that n=1anp\sum_{n=1}^{\infty} a_{n}^{p} is convergent for pCp \in C and divergent for pDp \in D. Define Ck=C[1,k]C_{k}=C \cap[1, k] and Dk[1,k]D_{k} \cap[1, k]. Lemma. For every positive integer kk there exists a positive integer NkN_{k} and a sequence Xk=(xk,1,,xk,Nk)X_{k}=(x_{k, 1}, \ldots, x_{k, N_{k}}) of complex numbers with the following properties: (a) For pDkp \in D_{k}, we have j=1Nkxk,jp1|\sum_{j=1}^{N_{k}} x_{k, j}^{p}| \geq 1. (b) For pCkp \in C_{k}, we have j=1Nkxk,jp=0\sum_{j=1}^{N_{k}} x_{k, j}^{p}=0; moreover, j=1mxk,jp1k|\sum_{j=1}^{m} x_{k, j}^{p}| \leq \frac{1}{k} holds for 1mNk1 \leq m \leq N_{k}. Proof. First we find some complex numbers z1,zkz_{1} \ldots, z_{k} with j=1kzjp={0pCk1pDk\sum_{j=1}^{k} z_{j}^{p}= \begin{cases}0 & p \in C_{k} \\ 1 & p \in D_{k}\end{cases}. As is well-known, this system of equations is equivalent to another system σν(z1,,zk)=wν(ν=1,2,,k)\sigma_{\nu}(z_{1}, \ldots, z_{k})=w_{\nu}(\nu= 1,2, \ldots, k) where σν\sigma_{\nu} is the ν\nu th elementary symmetric polynomial, and the constants wνw_{\nu} are uniquely determined by the Newton-Waring-Girard formulas. Then the numbers z1,,zkz_{1}, \ldots, z_{k} are the roots of the polynomial zkw1zk1++(1)kwkz^{k}-w_{1} z^{k-1}+-\ldots+(-1)^{k} w_{k} in some order. Now let M=max1mk,pCkj=1mzjpM=\lceil\max_{1 \leq m \leq k, p \in C_{k}}|\sum_{j=1}^{m} z_{j}^{p}|\rceil and let Nk=k(kM)kN_{k}=k \cdot(k M)^{k}. We define the numbers xk,1,xk,Nkx_{k, 1} \ldots, x_{k, N_{k}} by repeating the sequence (z1kM,z2kM,,zkkM)(\frac{z_{1}}{k M}, \frac{z_{2}}{k M}, \ldots, \frac{z_{k}}{k M}) (kM)k(k M)^{k} times, i.e. xk,=zjkMx_{k, \ell}=\frac{z_{j}}{k M} if j(modk)\ell \equiv j(\bmod k). Then we have j=1Nkxk,jp=(kM)kj=1k(zjkM)p=(kM)kpj=1kzjp\sum_{j=1}^{N_{k}} x_{k, j}^{p}=(k M)^{k} \sum_{j=1}^{k}(\frac{z_{j}}{k M})^{p}=(k M)^{k-p} \sum_{j=1}^{k} z_{j}^{p} then from (1) the properties (a) and the first part of (b) follows immediately. For the second part of (b), suppose that pCkp \in C_{k} and 1mNk1 \leq m \leq N_{k}; then m=kr+sm=k r+s with some integers rr and 1sk1 \leq s \leq k and hence j=1mxk,jp=j=1kr+j=kr+1kr+s=j=1s(zjkM)pM(kM)p1k|\sum_{j=1}^{m} x_{k, j}^{p}|=|\sum_{j=1}^{k r}+\sum_{j=k r+1}^{k r+s}|=|\sum_{j=1}^{s}(\frac{z_{j}}{k M})^{p}| \leq \frac{M}{(k M)^{p}} \leq \frac{1}{k}. The lemma is proved. Now let Sk=N1,NkS_{k}=N_{1} \ldots, N_{k} (we also define S0=0S_{0}=0 ). Define the sequence (a) by simply concatenating the sequences X1,X2,X_{1}, X_{2}, \ldots: (a1,a2,)=(x1,1,,x1,N1,x2,1,,x2,N2,,xk,1,,xk,Nk,)(a_{1}, a_{2}, \ldots)=(x_{1,1}, \ldots, x_{1, N_{1}}, x_{2,1}, \ldots, x_{2, N_{2}}, \ldots, x_{k, 1}, \ldots, x_{k, N_{k}}, \ldots) aSk+j=xk+1,j(1jNk+1)a_{S_{k}+j}=x_{k+1, j} \quad(1 \leq j \leq N_{k+1}). If pDp \in D and kpk \geq p then j=Sk+1Sk+1ajp=j=1Nk+1xk+1,jp1|\sum_{j=S_{k}+1}^{S_{k+1}} a_{j}^{p}|=|\sum_{j=1}^{N_{k+1}} x_{k+1, j}^{p}| \geq 1. By Cauchy's convergence criterion it follows that anp\sum a_{n}^{p} is divergent. If pCp \in C and Su<nSu+1S_{u}<n \leq S_{u+1} with some upu \geq p then j=Sp+1nanp=k=p+1u1j=1Nkxk,jp+j=1nSu1xu,jp=j=1nSu1xu,jp1u|\sum_{j=S_{p}+1}^{n} a_{n}^{p}|=|\sum_{k=p+1}^{u-1} \sum_{j=1}^{N_{k}} x_{k, j}^{p}+\sum_{j=1}^{n-S_{u-1}} x_{u, j}^{p}|=|\sum_{j=1}^{n-S_{u-1}} x_{u, j}^{p}| \leq \frac{1}{u}. Then it follows that n=Sp+1anp=0\sum_{n=S_{p}+1}^{\infty} a_{n}^{p}=0, and thus n=1anp=0\sum_{n=1}^{\infty} a_{n}^{p}=0 is convergent.

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