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account at time t would be considered discontinuous, since it "jumps" at each point in time when money is deposited or withdrawn.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

    How to find the expected value of a continuous variable with pdf fy= y^(-2)?

    Homework Statement Find the expected value of a continuous variable y with pdf fy= alpha*y^-2, 0<y<infinity. I know it is the integral from zero to infinity of y*fy, but I don't know where to go from there. I'm then supposed to use the expected value to find the method of moments...
  2. B

    Is it possible to define a basis for the space of continuous functions?

    In analogy to vector spaces, can we define a set of "basis functions" from which any continuous function can be written as a (possibly infinite) linear combination of the basis functions? I know the trigonometric functions 1, sin(nx), cos(nx) can be used for monotonic continuous functions...
  3. A

    Electric field of a continuous charge distribution at any point

    I am given a continuous charge problem in which there is a non-conducting wire of legnth L lying along the y-axis and I am required to calculate the electric field at any point along the x-axis. I know how to compute the electric field of a continuous charge distribution at a given point, but...
  4. A

    MHB Fn converges Uniformly to f, prove that f is continuous

    If f_n : A\rightarrow R sequnce of continuous functions converges uniformly to f prove that f is continuous My work Given \epsilon > 0 fix c\in A want f is continuous at c |f(x) - f(c) | = |f(x) - f_n(x) + f_n(x) - f(c) | \leq |f(x) - f_n(x) | + |f_n(x) - f(c) | the first absolute...
  5. D

    Prove that f is continuous

    Homework Statement Suppose that f is an odd function satisfying \mathop {\lim }\limits_{x \to {0^ + }} f(x) = f(0). Prove that f(0)=0 and f is continuous at x=0. Homework Equations The Attempt at a Solution Since f is an odd function f(0) = - f(0) \Rightarrow f(0) = 0 Let...
  6. L

    Metrics on continuous functions question

    Hey guys, I have been working on the following question: http://imageshack.us/a/img407/4890/81345604.jpg For part a f and g are continuous on I => there exists e > 0 and t_0 s.t. 0<|{f(t) - g(t)} - {f(t_0) - g(t_0)}| < e using |a-b| >= |a| - |b|, |{f(t) - g(t)} -...
  7. K

    Find the constant k that will make this piecewise continuous.

    Homework Statement Find a value for the constant k that will make the function below continuous: f(x)=\frac{x-1}{x^2-1}\ \text{if}\ x<=0 f(x)=\frac{tankx}{2x}~\text{if}~x>0 Homework Equations The Attempt at a Solution I've tried the only solution I can think of, which is to...
  8. D

    Show that a mapping is continuous

    Homework Statement Show that the mapping f carrying each point (x_{1},x_{2},...,x_{n+1}) of E^{n+1}-0 onto the point (\frac{x_{1}}{|x|^{2}},...,\frac{x_{n+1}}{|x|^{2}}) is continuous. [b]2. Continuity theorems I am given. A transformation f:S->T is continuous provided that if p is a limit...
  9. D

    Continuous Map to Single Point: Clarifying Confusion

    Hi all, I need help with a paragraph of my book that I don't understand. It says: "the map sending all of ℝ^n into a single point of ℝ^m is an example showing that a continuous map need not send open sets into open sets". My confusion arising because I can't figure out how this map can be...
  10. K

    Showing composition of functions are uniformly continuous

    Showing the sum of functions are uniformly continuous Homework Statement Suppose f and g are uniformly continuous on an interval I. Prove f + g are uniformly continuous on I. Homework Equations The Attempt at a Solution Let ε >0 By definition, since f and g are uniformly...
  11. B

    Pointwise Convergence of Fourier Series for a continuous function

    Where is the fallacy in this "proof" that the Fourier series of f(x) converges to f(x) if f is continuous at x and has period 2π? (I read in Wikipedia that a counterexample had been provided). Start with the Dirichlet integral for the N-th partial sum of the (trigonometric) Fourier series...
  12. D

    Continuous mappings in topology.

    I am trying to understand the theorem: Let f:S->T be a transformation of the space S into the space T. A necessary and sufficient condition that f be continuous is that if O is any open subset of T, then its inverse image f^{-1}(O) is open in S. First off, I don't really understand what...
  13. L

    Continuous time signals - Causal signal given even part

    Homework Statement Suppose h(t) is a causal signal and has the even part h_e(t) given by: h_e(t)= t[u(t)-u(t-1)]+u(t-1) for t>0. Find h(t) for all tHomework Equations For an even function f(x), f(x) = f(-x) Also even functions can be expressed as x_e(t) = 1/2[x(t)+x(-t)]The Attempt at a...
  14. K

    Show supremum of an interval made by a continuous, increasing function

    Homework Statement Let f be an increasing function defined on an open interval I and let c ϵ I. Suppose f is continuous at c. Prove sup{f(x)|x ϵ I and x < c} = f(c) Homework Equations The Attempt at a Solution Since I is an open interval and c is not able to be an end point...
  15. F

    Looking for a and b to ensure continuous function

    Homework Statement Find values for a and b that ensure f is a continuous function if f(x) = ax + 2b if x ≤ 0 x2 +3a - b if 0 < x ≤ 1 2x - 5 if x > 1 Homework Equations The Attempt at a Solution ax + 2b = 2x...
  16. P

    Is the function defined, continuous and differentiable

    Homework Statement Graph the function defined by the following. B = {(r/r0)B0 for r ≤ r0 {(r0/r)B0 for r > r0 (a) Is B continuous at r = r0? yes no (b) Is B differentiable at r = r0? Homework Equations The Attempt at a Solution I'm not exactly sure what to do...
  17. fluidistic

    Characteristic function of a continuous random variable

    Homework Statement I must calculate the characteristic function as well as the first moments and cumulants of the continuous random variable f_X (x)=\frac{1}{\pi } \frac{c}{x^2+c^2} which is basically a kind of Lorentzian.Homework Equations The characteristic function is simply a Fourier...
  18. H

    Questions about a f:X->Y being a bijective continuous function.

    Homework Statement For my proof, it tells me that f:X->Y is bijective. I understand that it is one-to-one and onto, but I just want to be clear about this from a neighborhood (open subset by our def) standpoint. Just to be clear: if f is bijective continuous, then that means the for all open...
  19. P

    Find Values of c and f that make h continuous

    Homework Statement { 2x if x<1 h(x)= { cx^2+d if 1<=x<=2 { 4x if x>2 Homework Equations The Attempt at a Solution It tried taking the limit of 2x and cx^2+d at x->1 (from both sides) and set them equal to each other. I did the...
  20. K

    Showing a fucntion is continuous on an interval

    Homework Statement Use the definition of continuity to prove that the function f defined by f(x)=x^(1/2) is continuous at every nonnative number. Homework Equations Continuity in this text is defined as Let I be an interval, let f:I→ℝ, and let c be in I. The function f is continuous...
  21. P

    Find the variable to make the function continuous

    Homework Statement Find k so that the following function is continuous on any interval. j(x) = {k cos(x), x ≤ 0 {10ex − k, 0 < x Homework Equations The Attempt at a Solution I originally thought i had to check if the limits of both parts of the functions...
  22. J

    Finding all continuous functions with the property that g(x + y) = g(x) + g(y)

    Homework Statement Determine all continuous functions g: R -> R such that g(x + y) = g(x) + g(y) for all x, y \in \mathbf{R} The Attempt at a Solution g(x) = g(x + 0) = g(x) + g(0). Hence G(0) = 0. G(0) = g(x + -x) = g(x) + g(-x) = 0. Therefore g(x) = -g(-x). It seems obvious that the only...
  23. H

    Showing that the directional derivatives exist but f is not continuous

    Homework Statement It says: \displaystyle f:{{\mathbb{R}}^{2}}\to \mathbb{R} \displaystyle f\left( x,y \right)=\left\{ \begin{align} & 1\text{ if 0<y<}{{\text{x}}^{2}} \\ & 0\text{ in other cases} \\ \end{align} \right. Show that all the directional derivatives about (0,0) exist but f...
  24. H

    Proof about continuous function related to balls and sets

    Homework Statement Let \displaystyle f:{{\mathbb{R}}^{n}}\to \mathbb{R} a continuous function. Proove that: If \displaystyle f\left( p \right)>0 then there's a ball \displaystyle {{B}_{p}} centered at p such that \displaystyle \forall x\in {{B}_{p}} we have \displaystyle f\left( x...
  25. N

    Prove [itex]f(x)=\sqrt{x^{2}+1}[/itex] is uniformly continuous on the real line.

    Homework Statement Prove f(x)=\sqrt{x^{2}+1} is uniformly continuous on the real line. Homework Equations Lipschitz Condition: If there is a constant M such that |f(p) - f(q)| \leq M |p-q| for all p,q \in D, then f obeys the Lipschitz condition. Mean Value Theorem: Let f be continuous on...
  26. A

    Absolutely continuous r.v. vs. continuous r.v.

    "Absolutely continuous r.v." vs. "continuous r.v." I've recently come across the term "absolutely continuous random variable" in a book on measure theoretic probability. How am I supposed to distinguish between AC random variables and just continuous random variables?
  27. Alesak

    Don't understand continuous basis

    Hi, I'm beginning to learn QM, and I've never seen any treatment of vector spaces with infinite bases. Countable case is quite digestible, but uncountable just flies over my head. Can anyone recommend me place where to learn this more advanced part of linear algebra, with focus on stuff...
  28. S

    Eigenfunctions of Operators with Continuous Sprectra

    I'm self-studying Griffith's Intro to Quantum Mechanics, and on page 100 he makes the claim that the eigenfunctions of operators with continuous spectra are not normalizable. I can't see why this is necessarily true. Hopefully I am not missing something basic. Thanks in advance.
  29. G

    Proving Limit of Integral of Continuous f w/ Jordans Lemma

    Homework Statement Suppose that f is continuous and that there exist constants A,B ≥ 0 and k>1 such that |f(z)|≤A|z|−k for all z such that |z|>B. let CR denote the semicircle given by |z| = R, Re(z) ≥ 0. Prove that limR→∞∫f(z)dz=0 Homework Equations The Attempt at a Solution I...
  30. C

    What does it mean to be strictly-strictly continuous?

    What does it mean to be "strictly-strictly" continuous? I am unsure what it means to be "strictly-strictly" continuous. Is that the same thing as saying just "strictly" continuous? Here is the context: \alpha is a unital *-homomorphism from M(A) to \mathcal{L}(A) such that \alpha is...
  31. J

    Question about the continuous beta-spectrum

    The last days I have been thinking about the following question. How does standard QM explain the continuous spectrum in beta-decay? Why can the created electrons (and, hence, also the neutrinos) in beta-decay acquire any possible energy within a certain range as long as their sum conserves...
  32. T

    Does this condition imply f:R^2->R is continuous?

    Here's an interesting question--I've asked some faculty members around here and "off the top of their head" none of them knows the answer. My gut says "yes", but my gut sucks at math. So here's the statement: Suppose we have a function f:\mathbb{R}^2\to\mathbb{R}, with the property that for...
  33. N

    How to show that this function is continuous at 0?

    Homework Statement For all real numbers, f is a function satisfying |f(x)|<=|x|. Show that f is continuous at 0 Homework Equations The Attempt at a Solution Really stuck on this cause I'm confused with the absolute values on this function. I *think* to show this you have to...
  34. F

    Determining a functions this is continuous at 0

    Homework Statement f(x) = {2x2 + x +3, x < 0 \frac{3}{x + 1} x ≥ 0 The 2 should be wrapped as 1 with a { but do not know how to do that. Homework Equations The Attempt at a Solution I was wondering if the squeeze rule would be...
  35. C

    MHB Proving T is Continuous in a Complex Banach Space

    Let X be a complex Banach space and T in L(X,X) a linear operator. Assuming only that (T*f)(x)=f(Tx), where x in X and f in X* how can I prove that T is continuous?
  36. H

    Find the continuous branch cut of a complex logarythm

    Homework Statement Find the continuous branch cut of a complex logarythm for C\[iy:y=>0] One of the complex numbers, for example, is -4i Homework Equations I don´t understand what to do with the subset. How could I find the continuous branch cut in the subset? The Attempt at a...
  37. H

    Transform of a piecewise continuous function

    We know that the \mathcal L\{f(t)\} = \int^{\infty}_0 e^{-st}f(t) dt. Say we want to, for example, solve the following IVP: y'' + y = f(t) where f(t) = \begin{cases} 0 & 0 \leq t < \pi \\ 1 & \pi \leq t < 2\pi\\ 0 & 2\pi \leq t \end{cases} and y(0) = 0 , y'(0) = 0 We apply Laplace on both...
  38. B

    Integration by parts if f' ang g' are not continuous

    The Integration by Parts Theorem states that if f' and g' are continuous, then ∫f'(x)g(x)dx = f(x)g(x) - ∫f(x)g'(x)dx. My question is, are those assumptions necessary? For example, this holds even if only one of the functions has a continuous derivative (say f' is not continuous but g'...
  39. R

    Prove Continuous Function f at 1/√2

    if f : (0, 1]--> R is given by f(x) = 0 if x is irrational, and f(x) = 1/(m+n) if x = m/n in (0, 1] in lowest terms for integers m and n. How can i prove that this function is continuous at 1/√2?
  40. S

    Finding Discrete Counterpart of Continuous Variable Energy Term

    Dear Sir… I am looking for a discrete counter part of a continuous variable. the continuous version of energy term in a liquid crystal is given by [\vec{n}\cdot(\nabla\times\vec{n})]^2. This is a square of a dot product between a vector 'n' and its curl field. My question is what is the exact...
  41. E

    An example of a continuous function in L1 space with no limit at infinity

    Homework Statement I am trying to come up with a continuous function in L1[0,infinity) that doesn't converge to 0 as the function goes out to infinity. Homework Equations I am trying to show an example of an f in L1[0,infinity) (i.e. ∫abs(f) < infinity) where the limit as the function...
  42. M

    A question about limit of a continuous function

    I am trying to solve a question and I need to justify a line in which |lim(x-->0)(f(x))|≤lim(x-->0)|f(x)| where f is a continuous function. Any help?
  43. C

    Continuous - how can I combine these open sets

    continuous -- how can I combine these open sets Homework Statement let ##X,Y## be compact spaces if ##f \in C(X \times Y)## and ## \epsilon > 0## then ## \exists g_1,\dots , g_n \in C(X) ## and ## h_1, \dots , h_n \in C(Y) ## such that ##|f(x,y)- \Sigma _{k=1}^n g_k(x)h_k(y)| < \epsilon...
  44. S

    Continuous functions are borel

    Homework Statement Take f: (a,b) --> R , continuous for all x0in (a,b) and take (Ω = (a,b) , F = ( (a,b) \bigcap B(R)) where B(R) is the borel sigma algebra Then prove f is a borel function The Attempt at a Solution I know that continuity of f means that for all x in (a,b) and all...
  45. K

    Continuous system & Infinite d.o.f.

    Hi All. I may sound weird and I know I am wrong somewhere. But a little explanation would really help. A system with 1 degree of freedom(d.o.f) has 1 governing differential equation. Similarly a system with 2 d.o.f has 2 (coupled) differential equations and so on. But a continuous system has...
  46. M

    Find All Values of a for Continuous Function f on Real Numbers

    How do you find all the values of "a" such that f is continuous on all real numbers? Find all values of a such that f is continuous on \Re f(x)= x+1 if x\leq a x^2 if x>a I tried solving but i do not even know where to start! Please help!
  47. A

    Discrete samples into continuous signal

    A) Let us say that we have some arbitrary sequence of natural numbers. e.g. 1, 2, 7, 3, 17, 19. Is it possible to convert every finite and infinite sequence into some continuous function model, such as in Fourier theory? I know that it is possible to extract some discrete samples from a...
  48. M

    Find the Values for which the function is continuous

    Homework Statement Determine the values of k,L,m and n such that the following function g(x) is continuous and differentiable at all points Homework Equations 2x2-n if x<-2 mx+L if -2≤x<2 kx2+1 if x≥2 The Attempt at a Solution So I know that...
  49. B

    Proof involving means in continuous distributions

    I recall reading somewhere that the mean value of a continuous variable is situated at a point that acts as a fulcrum about which all other values are considered "weights". In other words, if we define the mean as μ = \int^{∞}_{-∞} x ρ(x) dx (where rho is the probability density) then...
  50. J

    Continuous random variable (supply and demand)

    Homework Statement In the winter, the monthly demand in tonnes, for solid fuel from a coal merchant may be modeled by the continuous random variable X with probability density function given by: f(x)=\frac{x}{30} 0≤x<6 f(x)=\frac{(12-x)^{2}}{180} 6≤x≤12 f(x)=0 otherwise (a)...
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