diff --git a/chapters/functions/inverse-functions-logarithm/media/10_2_When_the_inverse_exists.png b/chapters/functions/inverse-functions-logarithm/media/10_2_When_the_inverse_exists.png new file mode 100644 index 0000000..4f87da9 Binary files /dev/null and b/chapters/functions/inverse-functions-logarithm/media/10_2_When_the_inverse_exists.png differ diff --git a/chapters/functions/inverse-functions-logarithm/media/10_4_The_natural_logarithm.png b/chapters/functions/inverse-functions-logarithm/media/10_4_The_natural_logarithm.png new file mode 100644 index 0000000..4ef2b3e Binary files /dev/null and b/chapters/functions/inverse-functions-logarithm/media/10_4_The_natural_logarithm.png differ diff --git a/chapters/functions/inverse-functions-logarithm/reading/README.md b/chapters/functions/inverse-functions-logarithm/reading/README.md deleted file mode 100644 index 2721ff9..0000000 --- a/chapters/functions/inverse-functions-logarithm/reading/README.md +++ /dev/null @@ -1 +0,0 @@ -# Inverse Functions and the Logarithm diff --git a/chapters/functions/inverse-functions-logarithm/reading/read.md b/chapters/functions/inverse-functions-logarithm/reading/read.md new file mode 100644 index 0000000..d008775 --- /dev/null +++ b/chapters/functions/inverse-functions-logarithm/reading/read.md @@ -0,0 +1,246 @@ +# Inverse Functions and the Logarithm + +## Inverse Function + +If $f$ is a function, then the function $g$ is the inverse function of $f$ if + +$$g(f(x))=x$$ + +for all $x$ in which $f(x)$ can be calculated. + +### Details + +The inverse of a function $f$ is denoted by $f^{-1}$, i.e. + +$$f^{-1}(f(x))=x$$ + +### Examples + +:::info Example + +If $f(x) = x^2$ for $x<0$ then the function $g$, defined as $g(y)=\sqrt{y}$ for $y>0$, is not the inverse of $f$ since $g(f(x))=\sqrt{x^2} = |x| = -x$ for $x<0$. + +::: + +## When the Inverse Exists: The Domain Question + +Inverses do not always exist. +For an inverse of $f$ to exist, $f$ must be one-to-one, i.e. for each $x$, $f(x)$ must be unique. + +![Fig. 13](../media/10_2_When_the_inverse_exists.png) + +Figure: The function $f(x)=x^2$ does not have an inverse since $f(x)=1$ + +has two possible solutions $-1$ and $1$. + +### Examples + +:::info Example + +$f(x)=x^2$ does not have an inverse since $f(x)=1$ has two possible solutions -1 and 1. + +::: + +:::note Note + +Note that iff $f$ is a function, then the function $g$ is the inverse function of $f$, if $g(f(x)) = x$ for all calculated values of $x$ in $f(x)$. + +The inverse function of $f$ is denoted by $f^{-1}$, i.e. $f^{-1}(f(x)) = x$. + +::: + +:::info Example + +What is the inverse function, $f^{-1}$, of $f$ if $f(x) = 5 + 4x$. +The simplest approach is to write $y=f(x)$ and solve for $x$. + +With + +$$f(x) = 5 + 4x$$ + +we write + +$$y = 5 + 4x$$ + +which we can now rewrite as + +$$y - 5 = 4x$$ + +and this implies + +$$\displaystyle\frac{y-5}{4} = x$$ + +And there we have it, very simple: + +$$f^{-1}(f(x)) = \displaystyle\frac{y - 5}{4}$$ + +::: + +## The Base 10 Logarithm + +When $x$ is a positive real number in $x=10^y$, $y$ is referred to as the base 10 logarithm of x and is written as: + +$$y=\log_{10}(x)$$ + +or + +$$y=\log(x)$$ + +### Details + +If $\log (x) = a$ and $\log (y)=b$, then $x = 10^a$ and $y = 10^b$, and + +$$x \cdot y = 10^a \cdot 10^b = 10^{a+b}$$ + +so that + +$$\log(xy) = a+b$$ + +### Examples + +:::info Example + +$$ +\begin{aligned} + \log(100) &= 2 \\ + \log(1000) &= 3 +\end{aligned} +$$ + +::: + +:::info Example + +If + +$$\log(2) \approx 0.3$$ + +then + +$$10^y=2$$ + +::: + +:::note Note + +$$2^{10}=1024 \approx 1000 = 10^3$$ + +therefore + +$$2 \approx 10^{3/10}$$ + +so + +$$\log (2) \approx 0.3$$ + +::: + +## The Natural Logarithm + +A logarithm with $e$ as a base is referred to as the natural logarithm and is denoted as $\ln$: + +$$y=\ln(x)$$ + +if + +$$x=e^y=\exp(y)$$ + +Note that $\ln$ is the inverse of $\exp$. + +![Fig. 14](../media/10_4_The_natural_logarithm.png) + +Figure: The curve depicts the function $y=\ln(x)$ and shows that $\ln$ is the inverse of $\exp$. +Note that $\ln(1)=0$ and when $y=0$ then $e^0=1$. + +## Properties of Logarithm(s) + +Logarithms transform multiplicative models into additive models, i.e. + +$$\ln(a\cdot b) = \ln a + \ln b$$ + +### Details + +This implies that any statistical model, which is multiplicative becomes additive on a log scale, e.g. + +$$y = a \cdot w^b \cdot x^c$$ + +$$\ln y = (\ln a) + \ln (w^b) + \ln (x^c)$$ + +Next, note that + +$$ +\begin{aligned} + \ln (x^2) &= \ln (x \cdot x) \\ + &= \ln x + \ln x \\ + &= 2 \cdot \ln x +\end{aligned} +$$ + +and similarly $\ln (x^n) = n \cdot \ln x$ for any integer $n$. + +In general $\ln (x^c) = c \cdot \ln x$ for any real number c (for $x>0$). + +Thus the multiplicative model (from above) + +$$y=a \cdot w^b \cdot x^c$$ + +becomes + +$$y= (\ln a) + b \cdot \ln w + c \cdot \ln x$$ + +which is a linear model with parameters $(\ln a)$, $b$ and $c$. + +In addition, the log-transform is often variance-stabilizing. + +## The Exponential Function and the Logarithm + +The exponential function and the logarithms are inverses of each other + +$$x = e^y \leftrightarrow y = \ln{x}$$ + +### Details + +:::note Note + +Note the properties: + +$$\ln (x \cdot y) = \ln (x) + \ln (y)$$ + +and + +$$e^a \cdot e^b = e^{a+b}$$ + +::: + +### Examples + +:::info Example + +Solve the equation + +$$10e^{1/3x} + 3 = 24$$ + +for $x$. + +First, get the $3$ out of the way: + +$$10e^{1/3x} = 21$$ + +Then the $10$: + +$$e^{1/3x} = 2.1$$ + +Next, we can take the natural log of 2.1. +Since $\ln$ is an inverse function of $e$ this would result in + +$$\displaystyle\frac{1}{3}x = \ln(2.1)$$ + +This yields + +$$x = \ln(2.1) \cdot 3$$ + +which is + +$$\approx 2.23$$ + +:::