What is Continuous: Definition and 1000 Discussions

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account at time t would be considered discontinuous, since it "jumps" at each point in time when money is deposited or withdrawn.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. G

    Continuous Absorption of Energy by Chlorophyl

    hi i was thinking that after the laws of quantum mechanics, where all atoms and molecules have discrete energy states, things like continuous spectrums should be forbidden. but why are there still molecules like chlorophyl which show this property? i would think, that this means, that the...
  2. andrewkirk

    Particle in a box - why must wave function be continuous?

    I am teaching myself quantum mechanics and have just read the particle in a box explanation, which is the first derivation of a theoretical reason why only discrete energy levels are possible within certain bound scenarios. In Shankar, the argument uses a requirement that the wave function...
  3. pairofstrings

    Meaning of continuous frequency domain

    Homework Statement Discrete-time transforms What does it mean when it says this : These transforms have a continuous frequency domain: Discrete-time Fourier transform Z-transform What is the meaning of continuous frequency domain..?
  4. L

    Continuous absorption spectrum - why this happens?

    Continuous absorption spectrum -- why this happens? Homework Statement A pure green glass plate placed in the path of light, absorbs everything everything except green, similarly red glass plate absorbs everything except red. Homework Equations May i know the reason for this? Thanks...
  5. J

    Let X be a continuous random variable. What value of b minimizes E (|X-b|)? Giv

    Let X be a continuous random variable. What value of b minimizes E(|X-b|)? Giv Homework Statement Let X be a continuous random variable. What value of b minimizes E(|X-b|)? Give the derivation The Attempt at a Solution E(|X - b|) E[e - \bar{x}] = E(X) E(|E[e - \bar{x}] - b|)...
  6. L

    Continuous absorption spectrum

    Homework Statement While dealing with continuous absorption spectrum, my book depicts like this " A pure green glass plate when placed in the path of white light, absorbs everything except green and gives continuous absorption spectrum" Homework Equations The Attempt at a Solution...
  7. B

    Continuous Bijection f:X->X not a Homeo.

    Continuous Bijection f:X-->X not a Homeo. Hi, All: A standard example of a continuous bijection that is not a homeomorphism is the map f:[0,1)-->S^1 : x-->(cosx,sinx) ; for one, S^1 is compact, but [0,1) is not,so they cannot be homeomorphic to each other. Now, I wonder...
  8. B

    Discrete Fourier transform of sampled continuous signal

    Homework Statement Let a system that converts a continuos-time signal to a discrete-time signal. The input x(t) is periodic with period of 0.1 second. The Fourier series coefficients of x(t) are X_k = \displaystyle\left(\frac{1}{2}\right)^{|k|}. The ideal lowpass filter H(\omega) is equal to 0...
  9. J

    Defining and Understanding Continuous Unit Normal Fields on Orientable Surfaces

    So I've been reading about orientated surfaces lately, and I always see the definition that a surface S is orientable if it is possible to choose a unit normal vector n, at every point of the surface so that n varies continuously over S. However, what does "varies continuously" mean? I never...
  10. S

    Continuous formula for area of segment of a circle

    I'm writing a little program for generating some images, and at one point I need to calculate how much of a circle is on either side of a straight line that bisects the circle. The line is always vertical so it is easy to get the value of how much of a horizontal line segment within the circle...
  11. B

    Continuous, Onto Function: (0,1)x(0,1)->R^2

    Continuous, Onto Function: (0,1)x(0,1)-->R^2 Hi, All: Just curious about finding a continuous onto function from the open unit square (0,1)x(0,1) into R^2. All I can think is that the function must go to infinity towards the edges, because if it could be continued into the whole square...
  12. J

    Help for Dynamics Continuous Motion

    Can someone tell me how to obtain the figure in red circle? I've trying out on my own but can't get that.
  13. C

    Continuous EM fields vs. fixed freq photons

    I understand the classical view of EM fields as being (theoretically) continuous. What I don't quite get is how this can be reconciled with the QM view of photons coming only in fixed frequencies (The electromagnetic field may be thought of in a more 'coarse' way.). Is the number of possible...
  14. Rasalhague

    Continuous dual space and conjugate space

    I've been reading Ballentine, Chapter 1. Have I got this the right way around? Taking our inner product to be linear in its second argument and conjugate linear in its first, the (continuous?) conjugate space of a Hilbert space \cal{H} is the following set of linear functionals, each identified...
  15. L

    Proof that a function is continuous

    Prove that the function is continuous when f(x)=0 f(x)=x4-7x3+11x2+7x-12f(c)-\epsilon<f(x)<f(c)+\epsilon Limits maybe taken, however, we do not have the value for c in the limit equation.
  16. B

    Finding a combination discrete and continuous cdf to make a new cdf

    Homework Statement Let F(x)=\begin{cases} .25e^{x} & -\infty<x<0\\ .5 & 0\leq x\leq1\\ 1-e^{-x} & 1<x<\infty\end{cases}$. Find a CDF of discrete type, F_d(x) and of continuous type, F_c(x) and a number 0<a<1 such that F(x)=aF_d(x)+(1-a)F_c(x) Homework Equations The Attempt at a...
  17. A

    Limits and Continuous Functions problem

    Homework Statement Define the function at a so as to make it continuous at a. f(x)=\frac{4-x}{2-\sqrt{x}}; a = 4 Homework Equations \lim_{x \rightarrow 4} \frac{4-x}{2-\sqrt{x}} The Attempt at a Solution I cannot think of how to manipulate the denominator to achieve f(4), so I...
  18. B

    Continuous fractions for root 2

    Hi all, Could anyone guide me on the following prove √2 = 1+1/(2 + 1/(2+ 1/(2+ 1/(2+···))))
  19. J

    Power and energy from continuous to discrete

    The energy and the power contents of a signal x(t), denoted by E_x and P_x, respectively, are defined as (1) E_x = \int ^{\infty}_{-\infty} |x(t)|^2 dt (2) P_x = lim_{T\rightarrow \infty} \frac{1}{T} \int ^{T/2}_{-T/2} |x(t)|^2 dt Let us use the discrete time (sampled) signal, with sampling...
  20. N

    Finding a probability given joint p.d.f of the continuous random variables

    I'm having a trouble doing this kind of problems :S Lets try this for example: The joint p.d.f of the continuous random variable X and Y is: f(x,y)= (2y+x)/8 for 0<x<2 ; 1<y<2 now we're asked to find a probability, say P(X+Y<2) I know i have to double integrate but how do I choose my...
  21. F

    Continuous random variable (stats)

    The probability density function of the time customers arrive at a terminal (in minutes after 8:00 A.M) is f(x)= (e^(-x/10))/10 for 0 < x c) Determine the probability that: two or more customers arrive before 8:40 A.M among five that arrive at the terminal. Assume arrivals are...
  22. C

    Electric potential continuous at boundaries?

    Why is it that we assume electric potential to be continuous across boundaries in electrostatics problems (like, say we have a situation with concentric spheres with different equations for electric field across boundaries)? This is the case as far as I've seen at least. I am in introductory E&M...
  23. R

    Algebraic sum of continuous random variable probabilities

    Hi all, I have a question relating to the sum of continuous random variable probabilities that I hope you can help to answer. In any probability density function (pdf), dealing with discrete or continuous random variables, the sum of the probabilities of all possible events must equal 1...
  24. A

    Uniformly continuous function (sequence)

    Homework Statement (This is my first post and I'm not sure why the Tex code isn't working, sorry).Suppose fis a positive continuous function on [1,0] .For each natural numbern define a new functionF_n s.t. F_n(x) = \int_0^1 t^ne^{xn}f(t)dt (a) Prove that lim_{n\to\infty}F_n(x) = 0 for...
  25. G

    Transition from bound states to continuous states

    Transition from bound states to "continuous" states If I have the Hamiltonian for the Hydrogen atom and a perturbation given by a classical electric field (the kind of problems you get in an ordinary course about QM, no QFT involved), can I have a transition from a bound state (I intend a...
  26. G

    Continuous eigenstates vs discrete eigenstates

    "Continuous eigenstates" vs "discrete eigenstates" There's this thing that's bothering me: if I have an Hamiltonian with a discrete and continuous spectrum, every book I read on quantum mechanics says that eigenvectors of discrete eigenvalues are orthogonal in the "Kronecker sense" (their...
  27. D

    Understanding Continuity: When is a Function Continuous?

    Homework Statement You're simply given f(x)/g(x) and it asks, when is the function continuous? There was one that was definitely wrong, so I remember these remaining choices: a) It is continuous when f(x) and g(x) are defined b) " " when g(x) cannot equal 0 c) " " when g(x) is defined...
  28. M

    Solutions to continuous dynamical system

    Homework Statement Consider a linear system dx/dt = Ax of arbitrary size. Suppose x1(t) and x2(t) are solutions of the system. Is the sum x(t) = x1(t) + x2(t) a solution as well? How do you know? Homework Equations The Attempt at a Solution I have no idea how to go about this...
  29. reddvoid

    Discrete vs Continuous Time Impulse Signals

    Whats the difference between a discrete time impulse and a continuous time impulse signal ?
  30. C

    What is the Normal Distribution of X in Vehicle Speed Observation?

    a vehicle driver gauges the relative speed of the next vehicle ahead by observing the speed with which the image of the width of that vehicle varies. This speed is proportional to X, the speed of variation of the angle at which the eye subtends this width. According to P. Ferrani and others, a...
  31. B

    Continuous on an open interval?

    Homework Statement Is t^2, -2t and 2 continuous on an open interval? Homework Equations I have re read the theorems and explanations of how something is continuous but i still don't understand it. The Attempt at a Solution
  32. S

    How a Absolute value Function can be continuous?

    Hello friends, I am quite confused how an absolute function is called a continuous one. f(x) = |x| has no limit at x=0 , that is when x > 0 it has a limit +1 {+.1, +.01, +.001} and -1 when x <0 {-.1, -.01, -.001} that is the reason it's not differentiable (left and right side limits are not the...
  33. P

    Topology: Connectedness and continuous functions

    Could you please check the statement of the theorem and the proof? If the proof is more or less correct, can it be improved? Theorem Let be a topological space and be the discrete space. The space is connected if and only if for any continuous functions , the function is not onto...
  34. A

    Is a uniform limit of absolutely continuous functions absolutely continuous?

    I was reading a Ph.D. thesis this morning and came across the claim that "a uniform limit of absolutely continuous functions is absolutely continuous." Is this true? What about the sequence of functions that converges to the Cantor function on [0,1]? Each of those functions is absolutely...
  35. H

    Continuous Functions Homework: Examples & Justification

    Homework Statement Find an example of a continuous function f:R->R with the following property. For every epsilon >0 there exists a delta >0 such that |f(x)-f(y)| <epsilon whenever x,y e R with |x-y|<delta. Now find an example of a continuous function f:R->R for which this property does nto...
  36. H

    A function which is continuous on Z only

    I have spent ages on this final part of a question but don't seem to be going anywhere - any help would be greatly appreciated! Given a function f:R->R let X be the set of all points at which f is continuous. Find an example of a function defined on R which is continuous on Z only.
  37. G

    Particle in abox : continuous functions problem

    I was studying particle in a box from shankar and I couldn't get the following point. If V is infinite at for x > L/2 and x < L/2, so is double derivative of psi. Now Shankar mentions that it follows the derivative of psi has a finite jump. I am not able to get this point because according to my...
  38. S

    Is time still undoubtedly continuous?

    When you combine general relativity and quantum mechanics theory, does time become quantised? Or are there any theories where this is a possibility? We're doing both special relativity and quantum mechanics at the moment, in different modules, both lecturers make passing references to the...
  39. U

    Understanding Momentum in Continuous Mass Flow Problems

    Homework Statement From 2.2 Worked Examplehttp://ocw.mit.edu/courses/physics/8-01sc-physics-i-classical-mechanics-fall-2010/continuous-mass-flow/MIT8_01SC_coursenotes19.pdf" Emptying a Freight Car - A freight car of mass mc contains a mass of sand ms At t = 0 a constant horizontal force of...
  40. H

    Fourier Series: Is f(x) Even or Piecewise Continuous?

    Hello. I have to find the Fourier series for f(x) = 1 + cos(pi x / L). My question is about f(x) Is this function even? I plotted it out and it looks even. The question I am completing starts off by saying: assume that any function f for which f and its derivative are piecewise continuous...
  41. A

    Hopefully easy question about sups of continuous functions

    If f is a continuous functional on a normed space, do you have \sup_{\|x\| < 1} |f(x)| = \sup_{\|x\| = 1} |f(x)| If so, why? If not, can someone provide a counterexample?
  42. S

    F uniformly continuous -> finite slope towards infinity

    f uniformly continuous --> finite slope towards infinity Homework Statement Given f:R \rightarrow R uniformly continuous. Show that \limsup_{x\rightarrow \infty} \displaystyle|f(x)|/x<\infty i.e. \exists C \in R: \, |f(x)|\leq C|x| as x \rightarrow \pm \infty. Homework Equations The...
  43. M

    Asap help please~electric fields from continuous charge distribution?

    Hello guys, I tried to figure this out and I got my answer. I just want to check it. So would you guys please help me with it? Thank you! Here is the question: A nonconducting rod of length 2a has a charge Q uniformly distributed along it. Find the expression for x-component of the electric...
  44. D

    Topology: Subset of A Continuous Function

    Homework Statement Let f be a real-valued function defined and continuous on the set of real numbers. Which of the following must be true of the set S={f(c):0<c<1}? I. S is a connected subset of the real numbers. II. S is an open subset of the real numbers. III. S is a bounded subset of the...
  45. A

    If light is quantized, why are EM spectrum and Blackbody spectrum continuous?

    If light is quantized, and is given out in packets, why are the EM wave spectrum and the black body spectrum continuous? I am very confused, can someone offer some explanation? Any input is greatly appreciated.
  46. Fredrik

    Continuous functions that vanish at infinity

    I'm trying to understand the set C_0(X), defined here as the set of continuous functions f:X\rightarrow\mathbb C such that for each \varepsilon>0, \{x\in X|\,|f(x)|\geq\varepsilon\} is compact. (If you're having trouble viewing page 65, try replacing the .se in the URL with your country domain)...
  47. S

    Proving z^5 is uniformly continuous on unit ball

    Homework Statement let f be the function defined in the region |z|<1 , by f(z)=z^5. prove that f is uniformly continuous in |z|<1...where z is a complex number Homework Equations The Attempt at a Solution
  48. B

    Show the Cube root of x is uniform continuous on R.

    Homework Statement Let f(x)=x^{1/3} show that it is uniform continuous on the Real metric space. Homework Equations By def. of uniform continuity \forall\epsilon>0 \exists\delta>0 s.t for \forall x,y\in\Re where |x-y|<\delta implies |f(x)-f(y)|< \epsilon The Attempt at a Solution...
  49. D

    Proper continuous map is closed

    Homework Statement Prove a proper continuous function from R to R is closed. Homework Equations proper functions have compact images corresponding to compact preimages, continuous functions have open images corresponding to open preimages, in R compact sets are closed and bounded...
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