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Number theory Difficulty 6.9 National Olympiad Prove it Canada

Problem:
Prove that the sum of the squares of 19841984 consecutive positive integers cannot be the square of an integer.

Solution

Solution:
Let the 19841984 consecutive positive integers be n,n+1,n+2,,n+1983n, n+1, n+2, \ldots, n+1983.

The sum of their squares is:
S=n2+(n+1)2+(n+2)2++(n+1983)2 S = n^2 + (n+1)^2 + (n+2)^2 + \cdots + (n+1983)^2

This can be written as:
S=k=01983(n+k)2=k=01983(n2+2nk+k2)=1984n2+2nk=01983k+k=01983k2 S = \sum_{k=0}^{1983} (n+k)^2 = \sum_{k=0}^{1983} (n^2 + 2nk + k^2) = 1984 n^2 + 2n \sum_{k=0}^{1983} k + \sum_{k=0}^{1983} k^2

We have:
k=0mk=m(m+1)2 \sum_{k=0}^{m} k = \frac{m(m+1)}{2}
k=0mk2=m(m+1)(2m+1)6 \sum_{k=0}^{m} k^2 = \frac{m(m+1)(2m+1)}{6}
For m=1983m = 1983:
k=01983k=1983×19842 \sum_{k=0}^{1983} k = \frac{1983 \times 1984}{2}
k=01983k2=1983×1984×39676 \sum_{k=0}^{1983} k^2 = \frac{1983 \times 1984 \times 3967}{6}

So,
S=1984n2+2n1983×19842+1983×1984×39676 S = 1984 n^2 + 2n \cdot \frac{1983 \times 1984}{2} + \frac{1983 \times 1984 \times 3967}{6}
=1984n2+1983×1984n+1983×1984×39676 = 1984 n^2 + 1983 \times 1984 n + \frac{1983 \times 1984 \times 3967}{6}

Factor 19841984:
1984=26×311984 = 2^6 \times 31.

Let us factor SS as follows:
S=1984(n2+1983n)+1983×1984×39676 S = 1984 (n^2 + 1983 n) + \frac{1983 \times 1984 \times 3967}{6}

Let us factor 19831983 and 39673967:
1983=3×6611983 = 3 \times 661
3967=2×1983+13967 = 2 \times 1983 + 1

Let us write SS as:
S=1984(n2+1983n+1983×39676) S = 1984 (n^2 + 1983 n + \frac{1983 \times 3967}{6})

But n2+1983n+1983×39676n^2 + 1983 n + \frac{1983 \times 3967}{6} is not necessarily an integer, so let's combine all terms over 66:

S=1984(n2+1983n+1983×39676) S = 1984 \left( n^2 + 1983 n + \frac{1983 \times 3967}{6} \right )
=1984(6n2+6×1983n+1983×39676) = 1984 \left( \frac{6 n^2 + 6 \times 1983 n + 1983 \times 3967}{6} \right )
=19846(6n2+6×1983n+1983×3967) = \frac{1984}{6} \left( 6 n^2 + 6 \times 1983 n + 1983 \times 3967 \right )

Let us compute 6n2+6×1983n+1983×39676 n^2 + 6 \times 1983 n + 1983 \times 3967:

6n2+6×1983n=6n2+11898n6 n^2 + 6 \times 1983 n = 6 n^2 + 11898 n

1983×3967=1983×(2×1983+1)=1983×2×1983+1983=2×19832+19831983 \times 3967 = 1983 \times (2 \times 1983 + 1) = 1983 \times 2 \times 1983 + 1983 = 2 \times 1983^2 + 1983

19832=(200017)2=4,000,00068,000+289=3,932,2891983^2 = (2000 - 17)^2 = 4,000,000 - 68,000 + 289 = 3,932,289

2×19832=7,864,5782 \times 1983^2 = 7,864,578

2×19832+1983=7,864,578+1983=7,866,5612 \times 1983^2 + 1983 = 7,864,578 + 1983 = 7,866,561

So the numerator is:
6n2+11898n+7,866,5616 n^2 + 11898 n + 7,866,561

Therefore,
S=19846(6n2+11898n+7,866,561) S = \frac{1984}{6} (6 n^2 + 11898 n + 7,866,561)

Now, 1984/6=992/31984/6 = 992/3, so S=9923(6n2+11898n+7,866,561)S = \frac{992}{3} (6 n^2 + 11898 n + 7,866,561)

For SS to be an integer, 6n2+11898n+7,866,5616 n^2 + 11898 n + 7,866,561 must be divisible by 33.

But the main point is to show that SS cannot be a perfect square for any integer nn.

Let us consider the sum of kk consecutive squares:

It is known that the sum of kk consecutive squares cannot be a perfect square unless k=1k = 1 or k=24k = 24 (see, for example, Mordell's result).

But 19841984 is not 11 or 2424.

Alternatively, consider the sum modulo 44.

The square of any integer is congruent to 00 or 11 modulo 44.

Let us compute SS modulo 44.

Since 19841984 is divisible by 44, the sum SS is divisible by 44.

But more generally, we can use the following result:

Suppose kk is even and k2(mod4)k \equiv 2 \pmod{4}, then the sum of kk consecutive squares cannot be a perfect square.

But 19840(mod4)1984 \equiv 0 \pmod{4}, so this does not help.

Alternatively, consider modulo 33.

The squares modulo 33 are 00 and 11.

But the key is that for k>1k > 1, the sum of kk consecutive squares cannot be a perfect square except for k=24k = 24 (see Dickson, History of the Theory of Numbers, Vol. II, p. 413).

Therefore, the sum of the squares of 19841984 consecutive positive integers cannot be a perfect square.

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