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Number theory Difficulty 5.7 AIME, harder Prove it Austria

Prove that if
(n2+1)2k(44n3+11n2+10n+2)=Nm (n^2 + 1)^{2k} \cdot (44n^3 + 11n^2 + 10n + 2) = N^m
holds for some non-negative integer values of mm, nn, NN and kk, m=1m = 1 must hold.

Solution

Since the left side of the equation is certainly larger than 1, we first note that m>0m > 0 must certainly hold.

Now, we consider even values of nn. Since n2+11(mod4)n^2+1 \equiv 1 \pmod 4 and 44n3+11n2+10n+22(mod4)44n^3+11n^2+10n+2 \equiv 2 \pmod 4 are certainly true, we have Nm2(mod4)N^m \equiv 2 \pmod 4. If m>1m > 1, NmN^m is odd for any odd NN and divisible by 4 for any even NN, and it follows that m=1m = 1 must hold, as claimed.

Next, we consider odd values of nn. In this case we have 44n3+11n2+10n+23(mod4)44n^3 + 11n^2 + 10n + 2 \equiv 3 \pmod 4 and n2+12(mod4)n^2 + 1 \equiv 2 \pmod 4, and we see that the factor 2 is contained in NmN^m exactly 2k2^k times.

For k=0k=0 we obtain Nm=(n2+1)(44n3+11n2+10n+2)2(mod4)N^m = (n^2+1)(44n^3+11n^2+10n+2) \equiv 2 \pmod 4, and the same argument holds as for even values of nn.

For k>0k > 0, the exponent m>1m > 1 must be a divisor of the exponent kk of 2 in the prime decomposition of NmN^m, and therefore a power of 2. This means that (n2+1)2k(n^2 + 1)^{2k} is an mm-th power, this must also be the case for 44n3+11n2+10n+244n^3 + 11n^2 + 10n + 2, and this number must certainly be a perfect square. This is not possible, however, since we have established that this number is 3(mod4)\equiv 3 \pmod 4, and therefore certainly not a perfect square. This case is therefore not possible, and we see that m=1m = 1 must hold, as claimed. \square

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