Maths Olympiad Prep

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Algebra Difficulty 5.4 AIME, harder Prove it JBMO

Problem:
Find all positive integers nn for which 13+23++163+17n1^{3} + 2^{3} + \cdots + 16^{3} + 17^{n} is a perfect square.

Solution

Solution:
We have 13+23++163=(1+2++16)2=13621^{3} + 2^{3} + \cdots + 16^{3} = (1 + 2 + \cdots + 16)^{2} = 136^{2}.

So, 13+23++163+17n=1362+17n1^{3} + 2^{3} + \cdots + 16^{3} + 17^{n} = 136^{2} + 17^{n}.

We want 1362+17n136^{2} + 17^{n} to be a perfect square.

Let 1362+17n=k2136^{2} + 17^{n} = k^{2} for some integer kk.

Then k21362=17nk^{2} - 136^{2} = 17^{n}, so (k136)(k+136)=17n(k - 136)(k + 136) = 17^{n}.

Since 1717 is prime, both k136k - 136 and k+136k + 136 must be powers of 1717.

Let k136=17ak - 136 = 17^{a}, k+136=17bk + 136 = 17^{b}, with a<ba < b, a+b=na + b = n.

Then k+136(k136)=272=17b17ak + 136 - (k - 136) = 272 = 17^{b} - 17^{a}.

So 272=17a(17ba1)272 = 17^{a}(17^{b - a} - 1).

Now, 17a17^{a} divides 272272. Since 272=16×17272 = 16 \times 17, the possible values for aa are 00 and 11.

Case 1: a=0a = 0

Then 17b01=27217^{b - 0} - 1 = 272, so 17b=27317^{b} = 273, which is not possible.

Case 2: a=1a = 1

Then 171(17b11)=27217^{1}(17^{b - 1} - 1) = 272, so 17b11=1617^{b - 1} - 1 = 16, so 17b1=1717^{b - 1} = 17, so b1=1b - 1 = 1, b=2b = 2.

So a=1a = 1, b=2b = 2, n=a+b=3n = a + b = 3.

Therefore, the only solution is n=3n = 3.

Check:
13+23++163+173=1362+4913=18496+4913=23409=15321^{3} + 2^{3} + \cdots + 16^{3} + 17^{3} = 136^{2} + 4913 = 18496 + 4913 = 23409 = 153^{2}, which is a perfect square.

Thus, the only positive integer nn for which 13+23++163+17n1^{3} + 2^{3} + \cdots + 16^{3} + 17^{n} is a perfect square is n=3n = 3.

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