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Peter O.
Peter O.
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Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • ~~For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.~~For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • ~~For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.~~

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

added 4 characters in body
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Peter O.
Peter O.
  • 697
  • 5
  • 22

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • For~~For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.~~

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • ~~For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.~~

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

deleted 110 characters in body
Source Link
Peter O.
Peter O.
  • 697
  • 5
  • 22

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous $(r-1)$-th derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivatives up to the $(r-1)$-th derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f^{(r-1)}$$f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f^{(r-1)}(x) + f^{(r-1)}(y) - 2f^{(r-1)}((x+y)/2)| \le D\epsilon$$|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r)$ equals the maximum of $D$ and the value of $M(f, r-1)$ in question 1$M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous $(r-1)$-th derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivatives up to the $(r-1)$-th derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f^{(r-1)}$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f^{(r-1)}(x) + f^{(r-1)}(y) - 2f^{(r-1)}((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r)$ equals the maximum of $D$ and the value of $M(f, r-1)$ in question 1?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

Let $f:[0,1]\to [0,1]$ be continuous. Let—

$$B_n(f)(x)=\sum_{k=0}^n f(k/n) {n \choose k} x^k (1-x)^{n-k},$$

be the Bernstein polynomial of $f$ of degree $n$.

This question relates to the difference between two Bernstein polynomials of the same function $f(x)$, namely the question of finding a simple and explicit function $\phi(f, n)$ such that— $$|B_{2n}(f)(x) - B_{n}(f)(x)| \le \phi(f, n) \text{ whenever } 0 \le x \le 1. \tag{1}$$

I have not been able to find results answering this question. However:

  • Butzer (1953) showed a very similar result on a family of operators, one of which is $2 B_{2n}(f) - B_n(f)$, but without explicit error bounds.
  • For the polynomials $2x(1-x)$ and $2x^2(1-x)$, the left-hand side of $(1)$ appears to converge to 0 at the rate $O(1/n^2)$. I suspect this is the case whenever $f(x)$ has a continuous second derivative.

Motivation

Answers to this question may help solve another question of mine, on finding bounds on the expected value of a hypergeometric random variable.

Questions

Let $M(f, r)$ equal the maximum absolute value of $f$ and its derivatives up to the $r$-th derivative.

  1. Given that $f$ has a continuous second derivative, can $\phi(f, n)$ equal $CM(f, 2)/n^2$ and/or $C/n^2$? In either case, find an upper bound for $C$.

  2. Given that $f$ is continuous, what is an explicit upper bound for $\phi(f, n)$, perhaps given the modulus of continuity of $f$?

  3. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f$ has a Lipschitz-continuous derivative, and
    • $M(f, r)$ equals the maximum absolute value of $f$ and the Lipschitz constants of $f$ and its derivative?

  4. Does the same bound for $C$ as in question 1 hold if, instead—

    • $f'$ is in the Zygmund class, that is, for some constant $D\ge 0$ it has the property $|f'(x) + f'(y) - 2f'((x+y)/2)| \le D\epsilon$ for every $\epsilon>0$, whenever $x$ and $y$ are in $[0, 1]$ and $|x-y|\le\epsilon$, and
    • $M(f, r) = \max(D, \max |f|, \max |f'|)$?

It would be nice if the answers to questions 1 and 4 can easily carry over to the problem of finding an upper bound for $|B_{n+1}(f)(x) - B_{n}(f)(x)|$.

References

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Daniele Tampieri
Daniele Tampieri
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Peter O.
Peter O.
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