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Number theory Difficulty 5.7 AIME, harder Prove it

Example 5 Proof: The number 979797^{97} cannot be expressed as the sum of cubes of several consecutive integers. [2]{ }^{[2]}

Solution

Prove the sum of cubes formula:
13+23++(n1)3+n3=n2(n+1)24 1^{3}+2^{3}+\cdots+(n-1)^{3}+n^{3}=\frac{n^{2}(n+1)^{2}}{4} \text {. }

In fact, it is only necessary to prove that 979797^{97} cannot be expressed as the sum of cubes of several consecutive positive integers.
Assume there exist m,nNm, n \in \mathbf{N}, such that
9797=(m+1)3+(m+2)3++(n1)3+n3 97^{97}=(m+1)^{3}+(m+2)^{3}+\cdots+(n-1)^{3}+n^{3} \text {. }

Then 9797=(13+23++n3)97^{97}=\left(1^{3}+2^{3}+\cdots+n^{3}\right)-
(13+23++m3)=n2(n+1)24m2(m+1)24. \begin{array}{r} \left(1^{3}+2^{3}+\cdots+m^{3}\right) \\ =\frac{n^{2}(n+1)^{2}}{4}-\frac{m^{2}(m+1)^{2}}{4} . \end{array}

Thus, n2(n+1)2m2(m+1)2=4×9797n^{2}(n+1)^{2}-m^{2}(m+1)^{2}=4 \times 97^{97}.
Using the difference of squares formula, factorize to get
(n(n+1)+m(m+1))(n(n+1)m(m+1))=4×9797. \begin{array}{l} (n(n+1)+m(m+1))(n(n+1)-m(m+1)) \\ =4 \times 97^{97} . \end{array}

Set n(n+1)+m(m+1)=2×97s(sN+)n(n+1)+m(m+1)=2 \times 97^{s}\left(s \in \mathbf{N}_{+}\right), n(n+1)m(m+1)=2×97t(tN)n(n+1)-m(m+1)=2 \times 97^{t}(t \in \mathbf{N}).
Then n(n+1)m(m+1)n(n+1)-m(m+1)
=(n+m+1)(nm)=2×97t =(n+m+1)(n-m)=2 \times 97^{t} \text {. }

Consider two cases.
(1) {n+m+1=2×97rnm=97tr,(rN+,tr)\left\{\begin{array}{l}n+m+1=2 \times 97^{r} \\ n-m=97^{t-r}\end{array},\left(r \in \mathbf{N}_{+}, t \geqslant r\right)\right..

Solving, we get {m=2×97r97tr12,n=2×97r+97tr12.\left\{\begin{array}{l}m=\frac{2 \times 97^{r}-97^{t-r}-1}{2}, \\ n=\frac{2 \times 97^{r}+97^{t-r}-1}{2} .\end{array}\right.
Thus, n(n+1)+m(m+1)n(n+1)+m(m+1)
=(2×97r+97tr)2+(2×97r97tr)224=\frac{\left(2 \times 97^{r}+97^{t-r}\right)^{2}+\left(2 \times 97^{r}-97^{t-r}\right)^{2}-2}{4}
=972r+972t2r12=2×97s=97^{2 r}+\frac{97^{2 t-2 r}-1}{2}=2 \times 97^{s}.
Therefore, 2×972r+972t2r1=4×97s2 \times 97^{2 r}+97^{2 t-2 r}-1=4 \times 97^{s}.
Since s1,r1s \geqslant 1, r \geqslant 1, we have 2t2r=02 t-2 r=0 (otherwise, the left side of equation (1) cannot be divisible by 97).
Thus, 972r=2×97s97^{2 r}=2 \times 97^{s}, which is impossible.
(2) {n+m+1=97r,nm=2×97tr(rN+,tr)\left\{\begin{array}{l}n+m+1=97^{r}, \\ n-m=2 \times 97^{t-r}\end{array}\left(r \in \mathbf{N}_{+}, t \geqslant r\right)\right..
Solving, we get {m=97r2×97tr12,n=97r+2×97tr12.\left\{\begin{array}{l}m=\frac{97^{r}-2 \times 97^{t-r}-1}{2}, \\ n=\frac{97^{r}+2 \times 97^{t-r}-1}{2} .\end{array}\right.
Thus, n(n+1)+m(m+1)n(n+1)+m(m+1)
=(97r+2×97tr)2+(97r2×97tr)224=972r12+972t2r=2×97s. \begin{array}{l} =\frac{\left(97^{r}+2 \times 97^{t-r}\right)^{2}+\left(97^{r}-2 \times 97^{t-r}\right)^{2}-2}{4} \\ =\frac{97^{2 r}-1}{2}+97^{2 t-2 r}=2 \times 97^{s} . \end{array}

Therefore, 972r1+2×972t2r=4×97s97^{2 r}-1+2 \times 97^{2 t-2 r}=4 \times 97^{s}.
Since s1,r1s \geqslant 1, r \geqslant 1, we have 2t2r=02 t-2 r=0 (otherwise, the left side of equation (2) cannot be divisible by 97).
Thus, 972r+1=2×97s97^{2 r}+1=2 \times 97^{s}, which is impossible.
In conclusion, 979797^{97} cannot be expressed as the sum of cubes of several consecutive positive integers.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.