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Number theory Difficulty 4.7 AIME Find the answer

Determine the remainder when 2122+2232++22011201222^{\frac{1 \cdot 2}{2}}+2^{\frac{2 \cdot 3}{2}}+\cdots+2^{\frac{2011 \cdot 2012}{2}} is divided by 7.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We have that 231(mod7)2^{3} \equiv 1(\bmod 7). Hence, it suffices to consider the exponents modulo 3. We note that the exponents are the triangular number and upon division by 3 give the pattern of remainders 1,0,0,1,0,0,1,0,0,1,0,0, \ldots, so what we want is 2122++220112012221+20+20+21++20+21(mod7)20103(21+20+20)+21(670)(4)+21\begin{aligned} 2^{\frac{1 \cdot 2}{2}}+\cdots+2^{\frac{2011 \cdot 2012}{2}} & \equiv 2^{1}+2^{0}+2^{0}+2^{1}+\ldots+2^{0}+2^{1} \quad(\bmod 7) \\ & \equiv \frac{2010}{3}\left(2^{1}+2^{0}+2^{0}\right)+2^{1} \\ & \equiv(670)(4)+2 \\ & \equiv 1 \end{aligned}

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