Questions tagged [piecewise-continuity]

For use when asking questions about piecewise continuous functions, their properties, or the functional branch itself.

Piecewise continuous functions are functions that have the following properties:

  1. The left and right hand limits exist everywhere on the function.

  2. The function is discontinuous for at least one point.

A piecewise continuous function can always be expressed as two functions f + g where f is continuous and g is a piecewise constant constant function.

g is sometimes referred to as the jump series or jump component of the piecewise continuous function. g can only be determined up to some constant c. The formula for finding g is:

$$g(x) = \sum_{n=0}^{JC(x)} (\lim_{a -> JL(n)^+} f(a) - \lim_{a -> JL(n)^-} f(a))$$

Where, JC(x) is the function counting the number of jump discontinuities between $0$ and x, and JL(x) is the discrete function returning the x coordinate of the X'th discontinuity. JC returns a negative count for values of negative x.

Another important operation involving piece-wise continuous functions are two alternative calculus operators, implied by the following:

  1. The indefinite integral "area function" always exists for piece-wise continuous functions, yet the anti-derivative doesn't always exist.

  2. There is no anti-indefinite integral such that the indefinite integral returns the original function, or some analogue fully defined for all $x \in R$

The result is two operators: the implied integral and the implied derivative which fulfill 1 and 2. The implied derivative is nothing more than the one sided derivative limit which is hardly useful or noteworthy. The implied integral on the other hand, has a much deeper change and is defined as a form similar to that of symbolic integration, but where any "symbol" of the form "$\lfloor f(x) \rfloor$" is held fixed and held fixed in the same manner that y is held fixed when integrating the multivariate function f(x,y) with respect to x. The implied integral will vary by piece wise constant functions of x, rather than the usual constants normal integration varies by.

The first fundamental theorem of implied calculus states:

Any integral of f is equal to the implied integral of f minus the jump series of the implied integral of f.

In this way, many integrals can be changed into a problem of finding the jump series.

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Formula for bump function

I would like to formulate a bump function (link) $f:\Bbb R \to\Bbb R$ with the following properties on the reals: $$ f(x) = \begin{cases} 0, & \mbox{if } x \le -1 \\ 1, & \mbox{if } x = 0 \\ 0, & \mbox{if } x \ge 1 \end{cases} $$ In addition…
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Derivative/integral relationship appears to disprove the fundamental theorem of calculus!!!

Consider the floor function: $$f(x) = \lfloor x \rfloor$$ The indefinite integral of f is: $$\int_0^x f(x) dx = x\lfloor x \rfloor - \frac {\lfloor x \rfloor^2 + \lfloor x \rfloor} 2$$ This should be an antiderivative of floor, right? Nope! If you…
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Solve the equation $\lfloor x^2\rfloor-3\lfloor x \rfloor +2=0$

Solve the equation $$ \lfloor x^2\rfloor-3\lfloor x \rfloor +2=0 $$ where $\lfloor x\rfloor $ denotes floor function. My Attempt: Let $x = n+f$, where $n= \lfloor x \rfloor \in \mathbb{Z} $, $f=x-\lfloor x \rfloor = \{x\} $, and $0\leq f<1$. Then…
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Can every piecewise linear function be exactly realized as a neural network?

Can every continuous piecewise linear function $[-1,1]^k \rightarrow \mathbb{R}^n$ be written as a composition of the following building blocks: Affine map: $x \mapsto Ax + b$ for some matrix $A$ and vector $b$ Relu activation: $(x_1, x_2, ...)…
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If a function is continuous everywhere, but undefined at one point, is it still continuous?

This is a question regarding the definition of continuity. My understanding of continuity is that a function is continuous at a point when it holds that $$\lim_{x\to a^-}f(x) = f(a) = \lim_{x\to a^+}f(x) \quad \quad (1)$$ The book I'm currently…
Alec
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Is there a good way to simplify this expression?

The Short Version Is there a way to simplify this expression? $$ \left(\left(\left(d × \left(\frac{j}{2}\right)^2\right)^2 − \frac{1}{27} × \left(\frac{j^2}{2 s}\right)^6\right)^\frac{1}{2} + d × \left(\frac{j}{2}\right)^2\right)^\frac{1}{3} +…
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Why is it necessary to split the definite integral of a piecewise function into a sum

The second fundamental theorem of calculus (Newton-Leibniz) tells us that: If $f$ is a real-valued function on a closed interval $[a, b]$ and $F$ is an antiderivative of $f$ in $[a,b]$ s.t. $F'(x)=f(x)$, then $$\int_{a}^{b} f(t) dt = F(b)-F(a)$$ Say…
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Finding a tricky composition of two piecewise functions

I have a question about finding the formula for a composition of two piecewise functions. The functions are defined as follows: $$f(x) = \begin{cases} 2x+1, & \text{if $x \le 0$} \\ x^2, & \text{if $x > 0$} \end{cases}$$ $$g(x) = \begin{cases} -x, …
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Is the indefinite integral of a piecewise continuous function a continuous function?

I had looked around on the web and can't find much information related to the integration of piecewise continuous functions. Let's say we have a simple function $$f(x)= \begin{cases} 0 & x\leq 0 \\ x & 0\leq x\leq 1 \\ 0 &…
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Proof for Sturm Liouville eigenfunction expantion pointwise convergence theorem

In "Elementary Partial Differential Equation" by Berg and McGregor, the following theorem is given without proof: Let $f(x)$ be piecewise smooth on the interval $[a,b]$ and let $\{\varphi_n(x)\}$ be the eigenfunctions of a self-adjoint regular…
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What's the difference between continuous and piecewise continuous functions?

A continuous function is a function where the limit exists everywhere, and the function at those points is defined to be the same as the limit. I was looking at the image of a piecewise continuous function on the following page:…
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How can this function be continuous at all points?

I have an exercise in which I need to determine which functions are continuous for all points Note: $\lfloor x\rfloor$ is floor of $x$. Function I need to consider is $f(x)= \lfloor x\rfloor+\sqrt{x- \lfloor x\rfloor}$ How can I prove that this…
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Can a function have a inflection point at somewhere non-differentiable?

Imagine this function : $$ f(x)=\begin{cases} x^2,\quad x<0 \\ \sqrt{x},\quad x\ge0. \end{cases} $$ In my lecture, my professor told me $f(x)$ is not differentiable at $0$ but $f(x)$ has an inflection point at $x=0$. I get the idea that…
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Do piecewise continuous functions form an inner product space?

While studying Fourier series, I encountered the fact that The set of piecewise continuous functions along with the inner product $$\langle f,g \rangle=\int_0 ^{2\pi}f(x)g(x) \, \mathrm{d}x$$ form an inner product space. However, when trying to…
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Is a piecewise linear function always a sum of concave and convex functions?

If I take a piecewise linear function (piecewise affine) is it true that I can always write it as a sum of concave and convex functions? My understanding of this…
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