Superposition of of bump functions identically equal to 1.












2












$begingroup$


I am trying to create a superposition of bump functions that adds identically to 1. Specifically I am looking to add two bump functions, say $f(x)$, $h(x)$ and $g(x)$ so that if $I,J,L subset mathbb{R}$ are the support of $f,h$ and $g$ respectively then I need $|I|=|J|=|L|$ and $Icap L = emptyset$. Furthermore, I need
$$
f(x)+h(x)+g(x) = 1
$$

for all $x in J$.



I guess I am getting stuck since the only bump function I know of is the usual



$$
b(x) = expbigg(1-frac{1}{1-big(frac{x-c}{r}big)^2}bigg)
$$

where $c$ is the peak and $r$ is the radius. I have been trying to combine these to get what I want but I haven't been having a lot of success.



Cheers in advance for any help.










share|cite|improve this question









$endgroup$

















    2












    $begingroup$


    I am trying to create a superposition of bump functions that adds identically to 1. Specifically I am looking to add two bump functions, say $f(x)$, $h(x)$ and $g(x)$ so that if $I,J,L subset mathbb{R}$ are the support of $f,h$ and $g$ respectively then I need $|I|=|J|=|L|$ and $Icap L = emptyset$. Furthermore, I need
    $$
    f(x)+h(x)+g(x) = 1
    $$

    for all $x in J$.



    I guess I am getting stuck since the only bump function I know of is the usual



    $$
    b(x) = expbigg(1-frac{1}{1-big(frac{x-c}{r}big)^2}bigg)
    $$

    where $c$ is the peak and $r$ is the radius. I have been trying to combine these to get what I want but I haven't been having a lot of success.



    Cheers in advance for any help.










    share|cite|improve this question









    $endgroup$















      2












      2








      2





      $begingroup$


      I am trying to create a superposition of bump functions that adds identically to 1. Specifically I am looking to add two bump functions, say $f(x)$, $h(x)$ and $g(x)$ so that if $I,J,L subset mathbb{R}$ are the support of $f,h$ and $g$ respectively then I need $|I|=|J|=|L|$ and $Icap L = emptyset$. Furthermore, I need
      $$
      f(x)+h(x)+g(x) = 1
      $$

      for all $x in J$.



      I guess I am getting stuck since the only bump function I know of is the usual



      $$
      b(x) = expbigg(1-frac{1}{1-big(frac{x-c}{r}big)^2}bigg)
      $$

      where $c$ is the peak and $r$ is the radius. I have been trying to combine these to get what I want but I haven't been having a lot of success.



      Cheers in advance for any help.










      share|cite|improve this question









      $endgroup$




      I am trying to create a superposition of bump functions that adds identically to 1. Specifically I am looking to add two bump functions, say $f(x)$, $h(x)$ and $g(x)$ so that if $I,J,L subset mathbb{R}$ are the support of $f,h$ and $g$ respectively then I need $|I|=|J|=|L|$ and $Icap L = emptyset$. Furthermore, I need
      $$
      f(x)+h(x)+g(x) = 1
      $$

      for all $x in J$.



      I guess I am getting stuck since the only bump function I know of is the usual



      $$
      b(x) = expbigg(1-frac{1}{1-big(frac{x-c}{r}big)^2}bigg)
      $$

      where $c$ is the peak and $r$ is the radius. I have been trying to combine these to get what I want but I haven't been having a lot of success.



      Cheers in advance for any help.







      real-analysis functional-analysis






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Jan 18 at 6:54









      Jandré SnymanJandré Snyman

      17910




      17910






















          1 Answer
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          1












          $begingroup$

          So, what you need are some tools for creating a larger variety of bump functions. The most important tool? Convolution $f*g(x)=int_{mathbb{R}}f(t)g(x-t),dt$; if $f$ is $C^{infty}$ and $g$ is anything at all reasonable, $f*g$ is also $C^{infty}$. We don't need even one derivative in $f$ to make that work. A bump function is also compactly supported; for that, note that the support of $f*g$ is contained in the sum of the support of $f$ and the support of $g$ - so if $f$ is a bump function and $g$ is compactly supported, $f*g$ is a bump function.



          OK, now to the problem. We want three bump functions $f,h,g$ that sum to $1$ on an interval, such that the outer two have disjoint support. Their sum $s$ is, of course, also a bump function, so let's find that first.



          So, how do we get a constant out of the convolution? We convolve with a constant; $b*1(x)=int_mathbb{R}b$, independent of $x$. Obviously, we don't want this on the whole real line - but if, instead, we convolve with the characteristic function of an interval $U=(rho-gamma,rho+gamma)$ larger than the support $B=(r-c,r+c)$ of $b$, we'll get a bump function which is constant on some smaller interval, of length $|U|-|B|$. So then, let $$u(x)=begin{cases}left(int_{mathbb{R}}bright)^{-1}&xin U\0&text{otherwise}end{cases}$$
          and $s=b*u$. This $s$ is supported on $[r+rho-c-gamma,r+rho+c+gamma]$ and is equal to $1$ on $[r+rho-gamma+c,r+rho+gamma-c]$.



          Now we need to break that $s$ up into a sum $f+h+g$, supported on three sets $I,J,L$ respectively of equal size, with $I$ and $L$ disjoint, and $J$ contained in that middle set where the sum is $1$. The obvious thing to do is to split $u$ into three equal parts, each supported on an interval of length $frac23gamma$. The first, leading to $f$ and $I$, will be $left(int_{mathbb{R}}bright)^{-1}$ times the characteristic function of $(rho-gamma,rho-frac13gamma]$, so we will have $I=[r+rho-gamma-c,r+rho-frac13gamma+c]$. Similarly, $J=[r+rho-frac13gamma-c,r+rho+frac13gamma+c]$ and $L=[r+rho+frac13gamma-c,r+rho+gamma+c]$. For this all to work out, we need ($I$ and $L$ disjoint)
          $$r+rho-frac13gamma+c < r+rho+frac13gamma-c$$
          $$2c<frac23gamma$$
          and (lower endpoint of $J$ in the region where the sum is $1$)
          $$r+rho-gamma+c < r+rho-frac13gamma-c$$
          $$2c < frac23gamma$$
          and another for the upper endpoint of $J$, which will simplify to the same condition as the two we already have. Basically, this three-part decomposition works as long as each of the individual pieces is large enough to have its own region where the convolution is constant.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:08










          • $begingroup$
            Yes, the method would work for that as well.
            $endgroup$
            – jmerry
            Jan 18 at 10:11










          • $begingroup$
            Cheers mate!!! I wish I could double upvote your answer.
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:11











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          $begingroup$

          So, what you need are some tools for creating a larger variety of bump functions. The most important tool? Convolution $f*g(x)=int_{mathbb{R}}f(t)g(x-t),dt$; if $f$ is $C^{infty}$ and $g$ is anything at all reasonable, $f*g$ is also $C^{infty}$. We don't need even one derivative in $f$ to make that work. A bump function is also compactly supported; for that, note that the support of $f*g$ is contained in the sum of the support of $f$ and the support of $g$ - so if $f$ is a bump function and $g$ is compactly supported, $f*g$ is a bump function.



          OK, now to the problem. We want three bump functions $f,h,g$ that sum to $1$ on an interval, such that the outer two have disjoint support. Their sum $s$ is, of course, also a bump function, so let's find that first.



          So, how do we get a constant out of the convolution? We convolve with a constant; $b*1(x)=int_mathbb{R}b$, independent of $x$. Obviously, we don't want this on the whole real line - but if, instead, we convolve with the characteristic function of an interval $U=(rho-gamma,rho+gamma)$ larger than the support $B=(r-c,r+c)$ of $b$, we'll get a bump function which is constant on some smaller interval, of length $|U|-|B|$. So then, let $$u(x)=begin{cases}left(int_{mathbb{R}}bright)^{-1}&xin U\0&text{otherwise}end{cases}$$
          and $s=b*u$. This $s$ is supported on $[r+rho-c-gamma,r+rho+c+gamma]$ and is equal to $1$ on $[r+rho-gamma+c,r+rho+gamma-c]$.



          Now we need to break that $s$ up into a sum $f+h+g$, supported on three sets $I,J,L$ respectively of equal size, with $I$ and $L$ disjoint, and $J$ contained in that middle set where the sum is $1$. The obvious thing to do is to split $u$ into three equal parts, each supported on an interval of length $frac23gamma$. The first, leading to $f$ and $I$, will be $left(int_{mathbb{R}}bright)^{-1}$ times the characteristic function of $(rho-gamma,rho-frac13gamma]$, so we will have $I=[r+rho-gamma-c,r+rho-frac13gamma+c]$. Similarly, $J=[r+rho-frac13gamma-c,r+rho+frac13gamma+c]$ and $L=[r+rho+frac13gamma-c,r+rho+gamma+c]$. For this all to work out, we need ($I$ and $L$ disjoint)
          $$r+rho-frac13gamma+c < r+rho+frac13gamma-c$$
          $$2c<frac23gamma$$
          and (lower endpoint of $J$ in the region where the sum is $1$)
          $$r+rho-gamma+c < r+rho-frac13gamma-c$$
          $$2c < frac23gamma$$
          and another for the upper endpoint of $J$, which will simplify to the same condition as the two we already have. Basically, this three-part decomposition works as long as each of the individual pieces is large enough to have its own region where the convolution is constant.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:08










          • $begingroup$
            Yes, the method would work for that as well.
            $endgroup$
            – jmerry
            Jan 18 at 10:11










          • $begingroup$
            Cheers mate!!! I wish I could double upvote your answer.
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:11
















          1












          $begingroup$

          So, what you need are some tools for creating a larger variety of bump functions. The most important tool? Convolution $f*g(x)=int_{mathbb{R}}f(t)g(x-t),dt$; if $f$ is $C^{infty}$ and $g$ is anything at all reasonable, $f*g$ is also $C^{infty}$. We don't need even one derivative in $f$ to make that work. A bump function is also compactly supported; for that, note that the support of $f*g$ is contained in the sum of the support of $f$ and the support of $g$ - so if $f$ is a bump function and $g$ is compactly supported, $f*g$ is a bump function.



          OK, now to the problem. We want three bump functions $f,h,g$ that sum to $1$ on an interval, such that the outer two have disjoint support. Their sum $s$ is, of course, also a bump function, so let's find that first.



          So, how do we get a constant out of the convolution? We convolve with a constant; $b*1(x)=int_mathbb{R}b$, independent of $x$. Obviously, we don't want this on the whole real line - but if, instead, we convolve with the characteristic function of an interval $U=(rho-gamma,rho+gamma)$ larger than the support $B=(r-c,r+c)$ of $b$, we'll get a bump function which is constant on some smaller interval, of length $|U|-|B|$. So then, let $$u(x)=begin{cases}left(int_{mathbb{R}}bright)^{-1}&xin U\0&text{otherwise}end{cases}$$
          and $s=b*u$. This $s$ is supported on $[r+rho-c-gamma,r+rho+c+gamma]$ and is equal to $1$ on $[r+rho-gamma+c,r+rho+gamma-c]$.



          Now we need to break that $s$ up into a sum $f+h+g$, supported on three sets $I,J,L$ respectively of equal size, with $I$ and $L$ disjoint, and $J$ contained in that middle set where the sum is $1$. The obvious thing to do is to split $u$ into three equal parts, each supported on an interval of length $frac23gamma$. The first, leading to $f$ and $I$, will be $left(int_{mathbb{R}}bright)^{-1}$ times the characteristic function of $(rho-gamma,rho-frac13gamma]$, so we will have $I=[r+rho-gamma-c,r+rho-frac13gamma+c]$. Similarly, $J=[r+rho-frac13gamma-c,r+rho+frac13gamma+c]$ and $L=[r+rho+frac13gamma-c,r+rho+gamma+c]$. For this all to work out, we need ($I$ and $L$ disjoint)
          $$r+rho-frac13gamma+c < r+rho+frac13gamma-c$$
          $$2c<frac23gamma$$
          and (lower endpoint of $J$ in the region where the sum is $1$)
          $$r+rho-gamma+c < r+rho-frac13gamma-c$$
          $$2c < frac23gamma$$
          and another for the upper endpoint of $J$, which will simplify to the same condition as the two we already have. Basically, this three-part decomposition works as long as each of the individual pieces is large enough to have its own region where the convolution is constant.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:08










          • $begingroup$
            Yes, the method would work for that as well.
            $endgroup$
            – jmerry
            Jan 18 at 10:11










          • $begingroup$
            Cheers mate!!! I wish I could double upvote your answer.
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:11














          1












          1








          1





          $begingroup$

          So, what you need are some tools for creating a larger variety of bump functions. The most important tool? Convolution $f*g(x)=int_{mathbb{R}}f(t)g(x-t),dt$; if $f$ is $C^{infty}$ and $g$ is anything at all reasonable, $f*g$ is also $C^{infty}$. We don't need even one derivative in $f$ to make that work. A bump function is also compactly supported; for that, note that the support of $f*g$ is contained in the sum of the support of $f$ and the support of $g$ - so if $f$ is a bump function and $g$ is compactly supported, $f*g$ is a bump function.



          OK, now to the problem. We want three bump functions $f,h,g$ that sum to $1$ on an interval, such that the outer two have disjoint support. Their sum $s$ is, of course, also a bump function, so let's find that first.



          So, how do we get a constant out of the convolution? We convolve with a constant; $b*1(x)=int_mathbb{R}b$, independent of $x$. Obviously, we don't want this on the whole real line - but if, instead, we convolve with the characteristic function of an interval $U=(rho-gamma,rho+gamma)$ larger than the support $B=(r-c,r+c)$ of $b$, we'll get a bump function which is constant on some smaller interval, of length $|U|-|B|$. So then, let $$u(x)=begin{cases}left(int_{mathbb{R}}bright)^{-1}&xin U\0&text{otherwise}end{cases}$$
          and $s=b*u$. This $s$ is supported on $[r+rho-c-gamma,r+rho+c+gamma]$ and is equal to $1$ on $[r+rho-gamma+c,r+rho+gamma-c]$.



          Now we need to break that $s$ up into a sum $f+h+g$, supported on three sets $I,J,L$ respectively of equal size, with $I$ and $L$ disjoint, and $J$ contained in that middle set where the sum is $1$. The obvious thing to do is to split $u$ into three equal parts, each supported on an interval of length $frac23gamma$. The first, leading to $f$ and $I$, will be $left(int_{mathbb{R}}bright)^{-1}$ times the characteristic function of $(rho-gamma,rho-frac13gamma]$, so we will have $I=[r+rho-gamma-c,r+rho-frac13gamma+c]$. Similarly, $J=[r+rho-frac13gamma-c,r+rho+frac13gamma+c]$ and $L=[r+rho+frac13gamma-c,r+rho+gamma+c]$. For this all to work out, we need ($I$ and $L$ disjoint)
          $$r+rho-frac13gamma+c < r+rho+frac13gamma-c$$
          $$2c<frac23gamma$$
          and (lower endpoint of $J$ in the region where the sum is $1$)
          $$r+rho-gamma+c < r+rho-frac13gamma-c$$
          $$2c < frac23gamma$$
          and another for the upper endpoint of $J$, which will simplify to the same condition as the two we already have. Basically, this three-part decomposition works as long as each of the individual pieces is large enough to have its own region where the convolution is constant.






          share|cite|improve this answer









          $endgroup$



          So, what you need are some tools for creating a larger variety of bump functions. The most important tool? Convolution $f*g(x)=int_{mathbb{R}}f(t)g(x-t),dt$; if $f$ is $C^{infty}$ and $g$ is anything at all reasonable, $f*g$ is also $C^{infty}$. We don't need even one derivative in $f$ to make that work. A bump function is also compactly supported; for that, note that the support of $f*g$ is contained in the sum of the support of $f$ and the support of $g$ - so if $f$ is a bump function and $g$ is compactly supported, $f*g$ is a bump function.



          OK, now to the problem. We want three bump functions $f,h,g$ that sum to $1$ on an interval, such that the outer two have disjoint support. Their sum $s$ is, of course, also a bump function, so let's find that first.



          So, how do we get a constant out of the convolution? We convolve with a constant; $b*1(x)=int_mathbb{R}b$, independent of $x$. Obviously, we don't want this on the whole real line - but if, instead, we convolve with the characteristic function of an interval $U=(rho-gamma,rho+gamma)$ larger than the support $B=(r-c,r+c)$ of $b$, we'll get a bump function which is constant on some smaller interval, of length $|U|-|B|$. So then, let $$u(x)=begin{cases}left(int_{mathbb{R}}bright)^{-1}&xin U\0&text{otherwise}end{cases}$$
          and $s=b*u$. This $s$ is supported on $[r+rho-c-gamma,r+rho+c+gamma]$ and is equal to $1$ on $[r+rho-gamma+c,r+rho+gamma-c]$.



          Now we need to break that $s$ up into a sum $f+h+g$, supported on three sets $I,J,L$ respectively of equal size, with $I$ and $L$ disjoint, and $J$ contained in that middle set where the sum is $1$. The obvious thing to do is to split $u$ into three equal parts, each supported on an interval of length $frac23gamma$. The first, leading to $f$ and $I$, will be $left(int_{mathbb{R}}bright)^{-1}$ times the characteristic function of $(rho-gamma,rho-frac13gamma]$, so we will have $I=[r+rho-gamma-c,r+rho-frac13gamma+c]$. Similarly, $J=[r+rho-frac13gamma-c,r+rho+frac13gamma+c]$ and $L=[r+rho+frac13gamma-c,r+rho+gamma+c]$. For this all to work out, we need ($I$ and $L$ disjoint)
          $$r+rho-frac13gamma+c < r+rho+frac13gamma-c$$
          $$2c<frac23gamma$$
          and (lower endpoint of $J$ in the region where the sum is $1$)
          $$r+rho-gamma+c < r+rho-frac13gamma-c$$
          $$2c < frac23gamma$$
          and another for the upper endpoint of $J$, which will simplify to the same condition as the two we already have. Basically, this three-part decomposition works as long as each of the individual pieces is large enough to have its own region where the convolution is constant.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 18 at 9:54









          jmerryjmerry

          10.8k1225




          10.8k1225












          • $begingroup$
            Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:08










          • $begingroup$
            Yes, the method would work for that as well.
            $endgroup$
            – jmerry
            Jan 18 at 10:11










          • $begingroup$
            Cheers mate!!! I wish I could double upvote your answer.
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:11


















          • $begingroup$
            Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:08










          • $begingroup$
            Yes, the method would work for that as well.
            $endgroup$
            – jmerry
            Jan 18 at 10:11










          • $begingroup$
            Cheers mate!!! I wish I could double upvote your answer.
            $endgroup$
            – Jandré Snyman
            Jan 18 at 10:11
















          $begingroup$
          Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
          $endgroup$
          – Jandré Snyman
          Jan 18 at 10:08




          $begingroup$
          Thanks for the incredibly detailed answer, more than I thought I would get. Tell me, will this method be able to be extended to as many of these functions as I need on a given interval? Say I wanted to fill the interval (0,2pi) with functions like this each with support on an interval of length 1/k for some real k, and no function has support overlapping with more than two others?
          $endgroup$
          – Jandré Snyman
          Jan 18 at 10:08












          $begingroup$
          Yes, the method would work for that as well.
          $endgroup$
          – jmerry
          Jan 18 at 10:11




          $begingroup$
          Yes, the method would work for that as well.
          $endgroup$
          – jmerry
          Jan 18 at 10:11












          $begingroup$
          Cheers mate!!! I wish I could double upvote your answer.
          $endgroup$
          – Jandré Snyman
          Jan 18 at 10:11




          $begingroup$
          Cheers mate!!! I wish I could double upvote your answer.
          $endgroup$
          – Jandré Snyman
          Jan 18 at 10:11


















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