[Math] Establish the composite Simpson’s 3/8 rule from basic Simpson’s 3/8 rule over n subintervals and the error

integrationnumerical methods

Establish the composite Simpson's 3/8 rule from basic Simpson's 3/8 rule over n subintervals and the error formula.

$$I(f) = \int_a^bf(x) dx \ \approxeq \ \frac{3h}{8}\left(f(a) \ + \ 3f\left(\frac{2a+b}{3}\right) \ + \ 3f\left(\frac{a+2b}{3}\right) \ + f(b)\right)$$
where $3h=b-a$, and the error of approximation $-3/80*f''''(c)*h^5$

I've spent a long time trying to understand how to derive the following result:
$$
\int_a^b{f(x) dx} \approx \frac{3h}{8} \left[ f(x_0) + 3 \sum_{i=1}^{m}{\left(f(x_{3i-2})+f(x_{3i-1})\right)} + \ 2 \sum_{i=1}^{m-1}f(x_{3i}) + f(x_{3m}) \right]
$$

I would appreciate it a lot if someone could give me an explanation and how to derive its error term.

Best Answer

Once the the quadrature formula obtained by cubic interpolation of $f$ based at nodes $(a,f(a))$, $(a+h,f(a+h))$, $(a+2h,f(a+2+2h))$ and $(b,f(b))$ with $h=\frac{b-a}{3}$ and its error are established, notice that this formula can be rewritten as $$\int^b_af\approx\frac{3h}{8}\Big(f(a)+3f(a+h)+3f(a+2h)+f(b)\Big)$$ with error $e_h=-\frac{(b-a)^5}{6480}f^{(iv)}(c)$ for some $c\in (a, b)$.

Divide the interval $[a,b]$ in $3m$ pieces of same length $h=\frac{b-a}{3m}$. This defines a partition $x_k=a+\frac{b-a}{3m}$. On the each subinterval of the form $[x_{3k},x_{3(k+1)}]$, $k=0,\ldots ,m-1$ apply Simpson's rules based on cubic interpolation (the fist formula in the OP). That is $$\begin{align} \int^b_af=\sum^{m-1}_{k=0}\int^{x_{3(k+1)}}_{x_{3k}}f\tag{1}\label{one} \end{align}$$ and $$I_k:=\int^{x_{3(k+1)}}_{x_{3k}}f\approx\frac{3h}{8}\Big(f(x_{3k})+3f(x_{3k+1})+3f(x_{3k+2})+f(x_{3(k+1)})\Big)=A_k$$ Denoting the righthand side of \eqref{one} by $A$ we have that \begin{align} \frac{8}{3h}A&=\sum^{m-1}_{k=0}\Big(f(x_{3k})+3f(x_{3k+1})+3f(x_{3k+2})+f(x_{3(k+1)})\Big)\\ &=f(x_0)+\sum^{m-1}_{k=0}3\big(f(x_{3k+1})+f(x_{3k+2})\big) +2\sum^{m-1}_{k=1}f(x_{3k}) + f(x_{3m}) \end{align} The composite formula follows.

As for the error, each approximation $A_k$ to $I_k$ has error $-\frac{3}{80} h^5 f^{(iv)}(c_k)$ for some $c_k\in (x_{3k},x_{3(k+1)})$. The sum of all errors is $$E:=-\frac{3}{80}h^5\sum^{m-1}_{j=0}f^{(iv)}(c_k)$$ If we assume that $f\in\mathcal{C}^4([a,b])$, there there is $c\in(a, b)$ such that $f^{(iv)}(c)=\frac1m\sum^{m-1}_{k=0}f^{(iv)}(c_k)$ and so $$ E=-\frac{1}{80}(b-a)h^4f^{(iv)}(c) $$

With regards to the error in this quadrature, there are somewhat explicit formulas for the errors resulting from interpolation methods on evenly distributed nodes:

For $n\in\mathbb{N}$, $n\geq2$, the error of approximation to $\int^b_af$ in the quadrature method based on interpolating over the uniform partition $a_k=\frac{k}{n}(b-a)$, $0\leq k\leq n$, is

  1. If $n$ is even and $f\in C^{(n+2)}[a,b]$, $$E_n=\frac{K_n}{(n+2)!}f^{(n+2)}(\eta),\quad K_n=\int^b_a x\prod^n_{k=0}(x-x_k)\,dx$$ for some $a<\eta<b$,
  2. If $n$ is odd and $f\in C^{(n+1)}[a,b]$, $$E_n=\frac{K_n}{(n+1)!}f^{(n+1)}(\eta),\qquad K_n=\int^b_a\prod^n_{j=1}(x-x_j)\,dx$$

For Simpson's 3/8 rule, $n=3$ and so $K_3=\int^b_a (x-a)(x-\tfrac{2a+ b}{3})(x-\tfrac{a+2b}{3})(x-b)\,dx$

The simplification, though messy with pencil and paper, yields $E_3=-\frac{(b-a)^5}{6480}f^{(4)}(\eta)$ for some $a<\eta<b$.

Reference: Krylov, V. I. (translated by Stroud, A.), Approximate calculation of integrals, Dover Publications. 2005 pp 89-91

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