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I need to find a function $f(x)$ such that the following data points would fit on it: $$f(1) = 0 \\ f(2) = 0.5 \\ f(4) = 1.0 \\ f(8) = 1.5 \\ \cdots $$ and so on. So the pattern is every time $x$ doubles $f(x)$ increases by $0.5$.

What is the equation for $f(x)$?

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A function where multiplying the argument by a fixed factor results in adding a fixed constant to the output is (more or less by definition) a logarithmic function, not a quadratic function. Specifically, it must be a function of the form $f(x) = c \log x + d$ for some real numbers $c$ and $d$, which you can determine by plugging in a couple of given values.

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  • $\begingroup$ You are the man. The answer is f(x) = log(x)/2 $\endgroup$ – user1068636 Oct 20 '12 at 19:51
  • $\begingroup$ @user1068636 : $\log_4 x$ is consistent with what you wrote. It's $(\log x)/2$ provided "$\log$" means the base-$2$ logarithm. $\endgroup$ – Michael Hardy Oct 20 '12 at 21:44
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If you plot $f(x)$ (W|A), the graph should give you a clue that this is a logarithmic function. Also, you can consider the fact that what you have is a recurrence relation: $f(x) = f(x/2) + 0.5$, which you solve using a technique such as Master theorem. Or for example using W|Alpha.

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