Polynomial Coefficient Calculation 3
Let $ P(x) $ be a polynomial of degree 2011 such that $ P(1) = 0, $ $ P(2) = 1, $ $ P(4) = 2, $ $ \dots, $ $ P(2^{2011}) = 2011 $. Then the coefficient of $ x $ in $ P(x) $ can be expressed in the form
\[a - \frac{1}{b^c},\]where $ a, $ $ b, $ $ c $ are positive integers, and $ b $ is prime. Find $ a + b + c $.
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- $\frac{a}{b}$
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- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
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- $\sin{}$
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- $\cap$
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Solution
We have that $ P(2^n) = n $ for $ 0 \le n \le 2011 $.
Let $ Q(x) = P(2x) - P(x) - 1 $. Then
\begin{align*}
Q(2^n) &= P(2^{n + 1}) - P(2^n) - 1 \\
&= n + 1 - n - 1 \\
&= 0
\end{align*}for $ 0 \le n \le 2010 $. Since $ Q(x) $ has degree 2011,
\[Q(x) = c(x - 1)(x - 2)(x - 2^2) \dotsm (x - 2^{2010})\]for some constant $ c $.
Also, $ Q(0) = P(0) - P(0) = -1 $. But
\[Q(0) = c(-1)(-2)(-2^2) \dotsm (-2^{2010}) = -2^{1 + 2 + \dots + 2010} c = -2^{2010 \cdot 2011/2} c,\]so $ c = \frac{1}{2^{2010 \cdot 2011/2}}, $ and
\[Q(x) = \frac{(x - 1)(x - 2)(x - 2^2) \dotsm (x - 2^{2010})}{2^{2010 \cdot 2011/2}}.\]Let
\[P(x) = a_{2011} x^{2011} + a_{2010} x^{2010} + \dots + a_1 x + a_0.\]Then
\[P(2x) = 2^{2011} a_{2011} x^{2011} + 2^{2010} a_{2010} x^{2010} + \dots + 2a_1 x + a_0,\]so the coefficient of $ x $ in $ Q(x) $ is $ 2a_1 - a_1 = a_1 $. In other words, the coefficients of $ x $ in $ P(x) $ and $ Q(x) $ are the same.
We can write $ Q(x) $ as
\[Q(x) = (x - 1) \left( \frac{1}{2} x - 1 \right) \left( \frac{1}{2^2} x - 1 \right) \dotsm \left( \frac{1}{2^{2010}} x - 1 \right).\]The coefficient of $ x $ in $ Q(x) $ is then
\begin{align*}
1 + \frac{1}{2} + \frac{1}{2^2} + \dots + \frac{1}{2^{2010}} &= \frac{1 + 2 + 2^2 + \dots + 2^{2010}}{2^{2010}} \\
&= \frac{2^{2011} - 1}{2^{2010}} \\
&= 2 - \frac{1}{2^{2010}}.\end{align*}The final answer is then $ 2 + 2 + 2010 = \boxed{2014} $.