Talk:Bump function

I'm not very expert C^\infty_0 may or may not be Frechet space, depending upon the topology we are considering...for example with the uniform convergence topology it is not metrizable, and that is the topology used to define distributions, so is misleading to say that c^\infty_o is a frechèt space.

The statement "C^infty_0 is a frechet space" is meaningless unless we specify the topology we are using; indeed interesting topologies on this space does not make it into a frechet space. Usually two topologies are used: the first one, the uniform convergence topology, is metrizable but not complete. The other one, the topology introduced by schwartz, is complete (in a generalized sense) but not metrizable, and is the one used to define distributions. In neither case C_0^\infty is a Frechet space. Of course one can also think to that space with the sup norm but that is quite unnatural since immediately afterwards the article claims its dual is the space of distributions, so i argue we are considering the schwartz topology. —Preceding unsigned comment added by 193.204.167.87 (talk) 15:28, 22 November 2007 (UTC)

Example

Correct me if I'm wrong, but the piecewise example in the article is not continuous, and does not match the graph. By l'hopital, the limit at 1 or -1 is one, because "1/(x^2-1)" converges to 0 at 1. (The limit equals "0/2x = 0"). This can be fixed by simply subtracting 1 from that secton of the function. Correct? 72.57.117.231 (talk) 16:11, 6 April 2010 (UTC)

Nevermind. 1/(x^2-1) is not indeterminate, so l'hopital doesn't apply. 72.57.117.231 (talk) 16:12, 6 April 2010 (UTC)

Isn't the space of n-dimensional bump functions actually an algebra?

The space $C_{0}^{\infty }(\mathbf {R} ^{n})$ of n-dimensional bump functions is closed not only under addition and scalar multiplication, but also under the multiplication of its elements by each other. This makes it into an algebra. Shouldn't this be mentioned in the article?50.203.182.230 (talk) 18:18, 3 June 2020 (UTC)

Rendering of Bump function expression in Chrome on Windows appears wrong

the negative sign shows up as a blank. — Preceding unsigned comment added by 207.140.43.75 (talk) 05:22, 23 January 2021 (UTC)

More examples

We only really have one example at the moment (we have one example in 1 dimension, which is used to construct one example in n dimensions) - some more would be nice. Joel Brennan (talk) 18:06, 13 July 2021 (UTC)

Alternative g(x)s

Are these things that could/should be added to the article, or would that be problematic/unhelpful?$g(x)={\frac {1+\tanh \sinh(2\tanh ^{-1}(|x|-|x-1|))}{2}}={\begin{cases}0&x\leq 0\\{\frac {1+\tanh \sinh(2\tanh ^{-1}(2x-1))}{2}}&0\leq x\leq 1\\1&1\leq x\end{cases}}={\begin{cases}0&x\leq 0\\{\frac {1}{1+e^{\frac {2x-1}{x^{2}-x}}}}&0<x<1\\1&1\leq x\end{cases}}$ 76.121.231.9 (talk) 04:52, 4 October 2023 (UTC)

Example of a true smooth bump function with straight-line slopes

A true example of a smooth bump function with straight-line slopes could be given by the integral

$q(x)=\int \limits _{-1}^{x}u(y)dy$

and its argument is given by the function:

$u(x)={\begin{cases}0,\quad |x|\geq 1\\0,\quad x=0\\2,\quad x=-{\frac {1}{2}}\\-2,\quad x={\frac {1}{2}}\\{\dfrac {-2\,{\text{sgn}}(x)}{1+\exp \left({\frac {1-2||2x|-1|}{(|2x|-1)^{2}-||2x|-1|}}\right)}},\quad {\text{otherwise}}\end{cases}}$

By construction the derivative has Flat function behavior at the points $|x|={\frac {1}{2}}$ showing $q(x)$ is a straight line at these points. 45.181.122.234 (talk) 21:57, 7 October 2024 (UTC)