Continuous Definition and 1000 Threads

In mathematics, a continuous function is a function that does not have any abrupt changes in value, known as discontinuities. More precisely, a function is continuous if arbitrarily small changes in its output can be assured by restricting to sufficiently small changes in its input. If not continuous, a function is said to be discontinuous. Up until the 19th century, mathematicians largely relied on intuitive notions of continuity, during which attempts such as the epsilon–delta definition were made to formalize it.
Continuity of functions is one of the core concepts of topology, which is treated in full generality below. The introductory portion of this article focuses on the special case where the inputs and outputs of functions are real numbers. A stronger form of continuity is uniform continuity. In addition, this article discusses the definition for the more general case of functions between two metric spaces. In order theory, especially in domain theory, one considers a notion of continuity known as Scott continuity. Other forms of continuity do exist but they are not discussed in this article.
As an example, the function H(t) denoting the height of a growing flower at time t would be considered continuous. In contrast, the function M(t) denoting the amount of money in a bank account at time t would be considered discontinuous, since it "jumps" at each point in time when money is deposited or withdrawn.

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  1. D

    Find f s. t. ||f||=1 and f(x) < 1 with ||x||=1

    Let ##f## be a continuous function defined in ##\mathbb{R}^n##. ##||\cdot ||## is the standard Euclidean metric. Then here are my suggested ways to choose ##f##: 1. Choose any continuous ##f## that satisfies $$1=\sup_{||x||\leq 1}||f||\neq \max_{||x||\leq 1}||f||$$ because the inequality...
  2. L

    Continuous functions on metric spaces part 2

    Hi, The task is as follows For the proof I wanted to use the boundedness, in the script of my professor the following is given, since both ##(X,d)## and ##\mathbb{R}## are normalized vector spaces I have now proceeded as follows ##|d(x,p)| \le C |x|## according to Archimedes' principle, a...
  3. D

    B Finite linear combination of continuous functions is continuous?

    ##G## and ##H## are real valued Lipschitz continuous functions. There exists a ##K_1,K_2\geq 0## such that for all ##s,t##, $$(s-t)^2\leq K_1^2 (G(s)-G(t))^2$$ and $$(s-t)^2\leq K_2^2 (H(s)-H(t))^2.$$ Is ##aG(t)+bH(t)## where ##a,b## are real constants also Lipschitz continuous? I tried showing...
  4. F

    Wavelet transform (CWT and DWT)

    Hello, I recently got interested in wavelets. The main idea seems clear: we compute the inner product between the signal ##x(t)## and a chosen wavelet for different scale factors and translations of the wavelet over the signal. The inner product provides the coefficient for a wavelet with a...
  5. Mashiro

    I Expressing any given point on plane with one unique number

    Currently, as far as I know, the two main ways to express any given point on a plane is through either cartesian plane or polar coordinates. Both of which requires an ordered pair of two numbers to express a point. However, I wonder if there exists such a system that could express any given...
  6. Euge

    POTW A Function in the Continuous Hölder Class

    Let ##0 < \alpha < 1##. Find a necessary and sufficient condition for the function ##f : [0,1] \to \mathbb{R}##, ##f(x) = \sqrt{x}##, to belong to the class ##C^{0,\alpha}([0,1])##.
  7. A

    I Is a Locally One-to-One Proper Map Globally Bijective?

    φ is a continuous, proper and locally one-to-one map.Is it a globally one-to-one map?
  8. Euge

    POTW Hölder Continuous Maps from ##R## to a Metric Space

    Let ##\gamma > 1##. If ##(X,d)## is a metric space and ##f : \mathbb{R} \to X## satisfies ##d(f(x),f(y)) \le |x - y|^\gamma## for all ##x,y\in \mathbb{R}##, show that ##f## must be constant.
  9. M

    Showing piece-wise function continuous

    For this, , The solution is, However, should they not write ##f(x) = \cos x## on ##[\frac{pi}{4}, \infty)## Many thanks!
  10. O

    I Is there a Boltzmann distribution for a system with continuous energy?

    Hi. I'm not sure where to put this question, thermodynamics or the quantum physics forum (or somewhere else). For a system in equillibrium with a heat bath at temperature T, the Boltzman distribution can be used. We have the probability of finding the system in state n is given by ##p_n =...
  11. M

    Proving g(x) is continuous over interval (-∞,-2)

    For number 18, The solution is, However, should they not write "For ## -∞ < a < -2##" since ##a ≠ -∞## (infinity is not a number)? Many thanks!
  12. M

    Confused about Continuous Endpoints: -1 < a < 1?

    For this problem, I don't understand why they are saying ##-1 < a < 1## since they are trying to find where ##f(x)## is continuous including the endpoints ##f(-1)## and ##f(1)## Why is it not: ##-1 ≤ a ≤1## Many thanks!
  13. M

    I Proving Continuous Functions in Smooth Infinitesimal Analysis

    Hello. How to prove that in smooth infinitesimal analysis every function on R is continuous? (Every function whose domain is R, the real numbers, is continuous and infinitely differentiable.) Thanks.
  14. D

    I Can a Satellite Maintain its Angular Velocity with Continuous Low Thrust?

    Suppose two satellites are in a circular heliocentric orbit with radius R and with angular velocity O'. Satellite 2 then undergoes a low continuous thrust. Can Satellite 2 (the one that undergoes the continuous low thrust) maintain the same angular velocity O' about the sun? It seems that...
  15. Euge

    POTW Uniformly Continuous Functions on the Real Line

    Let ##f : \mathbb{R} \to \mathbb{R}## be a uniformly continuous function. Show that, for some positive constants ##A## and ##B##, we have ##|f(x)| \le A + B|x|## for all ##x\in \mathbb{R}##.
  16. V

    Continuous Optimization, is this convex?

    f(x)=ln(|x1|+1)+(-2x1 2 +3x2 2 + 2x3 3) + sin(x1 + x2 + x3), for this problem in particular would be it be sufficient to find the Hessian and to see if that matrix is semi positive definite to determine if it convex?
  17. H

    I Partition function for continuous spectrum

    Let's say that we have a one-particle Hamiltonian that admits only a continuous spectrum of eigenvalues ##E(k)=\alpha k^2## parameterized by asymptotic momentum ##\mathbf{k}## (assuming the eigenfunctions become planewaves far from the origin), would the partition function then be $$Z=\int...
  18. Algr

    What continuous wattage can I get out of a 12v lantern battery?

    I have a C-Pap machine, and I'd like to put together something to power it for a few hours if we have a blackout. Wiring two of the above lantern batteries together should produce 24 volts. But I can't find any info on how much wattage you can draw from these before the voltage will sag. It...
  19. E

    B Is All Motion Discrete or Continuous in QM?

    We were discussing how much weight it would take to stop the mechanism from rotating in this thread: https://www.physicsforums.com/threads/weight-required-to-hang-straight-down-with-known-torque.1016470/#post-6646777 I wondered if there were actually a range of weights that would stop it...
  20. D

    Normalizing Wavefunction: Hard QM Question)

    So I have come up with my solution(attempt) which is: where ( $$\psi_ 1 \triangleq Asin(kx),0<x<L$$ $$\psi_ 2 \triangleq Be^{-sx},x>L$$ $$k \triangleq \sqrt{\frac{2mE}{\hbar^2}} $$ $$s \triangleq \sqrt{\frac{2m(V-E)}{\hbar^2}} $$) But this has a serious problem about boundary: I think...
  21. B

    MHB Understanding Continuous Functions: Examining f'(7) Undefined

    Suppose f is a function such that f'(7) is undefined. Which of the following statements is always true? (Give evidences that supports your answer, then explain how those evidences supports your answer) a. f must be continuous at x = 7. b. f is definitely not continuous at x = 7. c. There is not...
  22. K

    I Exploring Continuous Approximation of 1D Random Walk Steps (Reif, pg 14)

    Reif,pg 14. ##n_1## is the number of steps to the right in a 1D random walk. ##N## are the total number of steps "When ##N## is large, the binomial probability distribution ##W\left(n_{1}\right)## ##W\left(n_{1}\right)=\frac{N !}{n_{1} !\left(N-n_{1}\right) !} p^{n_{1}} q^{N-n_{1}}## tends to...
  23. T

    MHB Continuous, discontinuous and piece-wise function

    help me please to determine what are the equations i need tofinish my activity. Thankyou
  24. D

    Prove that f is a homeomorphism iff g is continuous, fg=1 and gf=1

    Outline of proof: Part I: ##1.## ##f## is a homeomorphism, so there exists a continuous inverse ##g:Y\longrightarrow X##. ##2.## ##f## is a bijection, hence there is a unique ##f(x)## in ##Y## for every ##x## in ##X##. For every ##f(x)\in Y##, the preimage under ##f## is...
  25. D

    Prove that if any f:X-->Y is continuous, X is the discrete topology

    Sketch of proof: ##1.## Let ##V## be open in ##Y##. ##2.## For arbitrary ##f:X\longrightarrow Y## and for arbitrary ##V##, ##f^{-1}(V)## is in ##X##. ##3.## ##f:X\longrightarrow Y## is continuous, so ##f^{-1}(V)## is open in ##X##. ##4.## Every subset ##f^{-1}(V)## of ##X## is open, so ##X##...
  26. D

    Prove that a product of continuous functions is continuous

    ##f## is continuou on ##\mathbb{C}##, so for al ##\epsilon>0##, there is a ##\delta>0## such that $$|\tilde{z}-z|\leq \delta \Rightarrow |f(\tilde{z})-f(z)|\leq \epsilon$$ for all ##\tilde{z}## and ##z## in ##\mathbb{C}##. Complex conjugation is a norm preserving operation on ##\mathbb{C}##, so...
  27. ktmsud

    I Continuous Spectrum and Energy levels of Electrons (Energy Bands)

    My book says that emission spectra are produced when an electron in excited state jump from excited to lower energy states. It also states that solids and liquids produce continuous spectra and it depends upon temperature only (is this black body radiation?). I know, Electrons around a nucleus...
  28. D

    Continuous joint random variable

    (a) $$\int_0^1\int_0^1x+cy^2 dxdy=\int_0^1 [\frac{x^2}{2}+cxy^2]_0^1dy= \int_0^1\frac{1}{2}+cy^2 dy=[\frac{y}{2}+\frac{cy^3}{3}]_0^1=\frac{1}{2}+\frac{c}{3}=1$$ $$\Rightarrow c=\frac{3}{2}$$ (b) The marginal pdf of X is $$f_X(a)=\int_0^1 f_{X,Y}(a,b)db=\int_0^1 x+\frac{3}{2}y^2...
  29. S

    MHB Interpolating Points with Continuous Modular Functions?

    Define a continuous function F(x;n) that interpolates points (x, x mod n) for a given integer n and all integer x. For example F(x;2)=\frac{1}{2}-\frac{1}{2}\cos\left(\pi x\right) interpolates all points (x, x mod 2) when x is an integer. Similarly F(x;3) should interpolate points (0,0), (1,1)...
  30. D

    Prove that the concatenation function is continuous

    Let f be continuous in [0,1] and g be continuous in [1,2] and f(1)=g(1). prove that $$ (f*g)= \begin{cases} f(t), 0\leq t\leq 1\\ g(t), 1\leq t \leq2 \end{cases}$$ is continuous using the universal property of quotient spaces. Let ##f:[0,1]→X## and ##g:[1,2]→Y## f and y are continuous, thus...
  31. Mikaelochi

    I Proving a function f is continuous given A U B = X

    Basically with this problem, I need to show that f is continuous if A and B are open and if A and B are closed. My initial thoughts are that in the first case X must be open since unions of open sets are open. My question is that am I allowed to assume open sets exist in Y? Because then I can...
  32. D

    I Is time continuous or discrete in quantum physics?

    I was working on plotting fidelity with time for two quantum states. First I used discrete time( t= 0,1,2,3...etc) to plot my fidelity. I got constant fidelity as 1 with continuous value of time. Next I used discrete set of values ( t=0 °,30 °,60 °,90 °). Here I saw my fidelity decreases and...
  33. crudux_cruo

    B Torque applied by a continuous mass instead of point particle

    I came across this 'problem' when I was trying to think about how a torsion spring would apply torque in something like a miniature catapult. I understand that in the context of something like turning a wrench, we can find the net torque on the wrench by treating the hand applying the force as...
  34. C

    Showing continuous function has min or max using Cauchy limit def.

    Problem: Let ## f: \Bbb R \to \Bbb R ## be continuous. It is known that ## \lim_{x \to \infty } f(x) = \lim_{x \to -\infty } f(x) = l \in R \cup \{ \pm \infty \} ##. Prove that ## f ## gets maximum or minimum on ## \Bbb R ##. Proof: First we'll regard the case ## l = \infty ## ( the case...
  35. M

    Probability: Prior Distribution for a Continuous Variable

    Hi, I was attempting this problem from the MIT OCW website probability and statistics course. Context: When Jane started class, she warned Jon that she tends to run late. Not just late, but uniformly late. That is, Jane will be late to a random class by ##X## hours, uniformly distributed over...
  36. Peter Jones

    The maximum efficiency of two continuous processes

    I think it reaches its maximum efficiency when it is two continuous Carnot process. Its efficiency then will be H= 1-(W1+W2)/(Q1+Q2), with W1/Q1>=T2/T1 and W2/Q2>=T3/T2 therefore H<= 1- (Q1.T2/T1+Q2.T3/T2)/(Q1+Q2), that is as far as i can go, have not got a result yet
  37. Eclair_de_XII

    B Are continuous functions on sequentially compact sets u-continuous?

    Suppose ##f## is not uniformly-continuous. Then there is ##\epsilon>0## such that for any ##\delta>0##, there is ##x,y\in K## such that if ##|x-y|<\delta##, ##|f(x)-f(y)|\geq \epsilon##. Choose ##\delta=1##. Then there is a pair of real numbers which we will denote as ##x_1,y_1## such that if...
  38. R

    Find a continuous solution to an ODE that includes a step function

    Non-homegenous first order ODE so start with an integrating factor ##\mu## $$\mu=\textrm{exp}\left(\int a dt\right)=e^t.$$ Then rewrite the original equation as $$\frac{d}{dt}\mu y = \mu g(t).$$ Using definite integrals and splitting the integration across the two cases, $$\begin{align}...
  39. B

    Show that f such that f(x+cy)=f(x)+cf(y) is continuous

    We need to show that ##\lim_{x \rightarrow a}f(x)=f(a), \forall a \in \mathbb{R}## . At first, I tried to show that f is continuous at 0 and from there I would show for all a∈R. But now, I think this may not even be true. I only got that f(0)=0. I'm very confused, I appreciate any help!
  40. R

    I Explaining the Continuous Diffraction Spectrum of a Heated Solid

    This is the figure from the book. First of all, from what I know about diffraction, there is an interference pattern but not dispersion of the different colors. If what is happening here can be explained that would be great. Second, the book says the line spectra for different gasses are due to...
  41. L

    MHB Is an increasing monotonic function in a closed interval also continuous?

    Hello all, Is this statement true ? Is every increasing monotonic function in a closed interval also continuous ? How do you prove such a thing ? Thank you !
  42. L

    MHB Is non continuous function also not Bounded ?

    Dear all, I am trying to figure out if a non continuous function is also not bounded. I know that a continuous function in an interval, closed interval, is also bounded. Is a non continuous function in a closed interval not bounded ? I think not, it makes no sense. How do you prove it ? Thank...
  43. T

    Determine whether a function is continuous or differentiable

    Perhaps use the definition of continuity, partial differentiability?
  44. F

    Engineering Continuous and discontinuous modes of series excited DC motors

    Here's my attempt: clc; clear all; Ra=1.8; La=150*10^(-3); Ls=150*10^(-3); Rs=1.8; Rt=Rs+Ra; ws=100*pi; c=1.25; vrms=240; vm=240*sqrt(2); f=50; T=1:0.5:50; for a=0:pi/9:pi/3 vt=(2*vm*cos(a))/pi wc=((ws*(Ls+La)/tan(a))-Rs-Ra)/c; x=[wc; a+pi]; Tc=c*(2*vm*cos(a)/(pi*(c*wc+Ra+Rs)))^2; for...
  45. LCSphysicist

    I About groups and continuous curves

    Define $$\phi(A)$$ a transformation which, acting on a vector x, returns $$AxA^{*}$$, in such way that if A belongs to the group $$SL(2,C)$$, $$||\phi(A)x||^2 = ||x||^2$$, so it conserves the metric and so is a Lorentz transformation. $$\phi(AB)x = (AB)x(AB)^{*} = ABxB^{*}A^{*} = A(BxB^{*})A^{*}...
  46. MidgetDwarf

    Is the given function continuous at x= 0? f(x) = {sin(1/x) if x≠0, 1

    I know that the function, g(x)= sin(1/x) has infinite oscillations when the values of x get closer and closer to 0. So its limit does not exist (from graphing it). However, the way that we defined f(x), at x=0, f(x)=1, but f(x)= sin(1/x) on (0,infinity). I have an issue in general showing that...
  47. K

    Photoelectric effect and continuous energy function

    E=hf-W where W is a work function. However we know that electrons in an atom will be excited only when radiated with photons of n*f0 discrete number of frequencies. where E=hf-W is a continuous function. Is this because energy level is continuous within a conductor? If we think of only...
  48. archaic

    Calculating a mean related to a continuous random variable

    I am not sure about how to approach this. Since the volume is uniformly distributed, the mean volume is ##(5.7+5.1)/2=5.4##, which is less than ##5.5##. From this, I could say that, on average, the producer won't spend any extra dollars. But then I thought that maybe I should interpret this as...
  49. Lynch101

    I Continuous vs discrete Universe

    I've come across the question of continuity vs discreteness in different articles, discussions, etc. but I'm not sure that I am 100% clear on what the precise question is. My basic interpretation of it is a question of whether the Universe is made up of lots of separate entities which all...
  50. cwill53

    Electric Field and Continuous Charge Distribution

    I sort of understand the meaning of this integral, but I don't know how to evaluate it. I have never evaluated a volume integral. It would be very helpful if someone could explain in other words what this integral means and give an example evaluating it. This is from Purcell's Electricity and...
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