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Finite Euler–Maclaurin formula for the composite trapezoid

Statement

Let a<b, let N≥1 and p≥1 be integers, put h=(b−a)/N and xi=a+ih, and let f∈C2p([a,b]) in the sense of Higher derivatives and the classes Ck and C∞. Write Bm(x) for Bernoulli polynomials by a normalized coefficient recursion and Bm:=Bm(0). For ui(x):=(x−xi)/h on [xi,xi+1], the composite trapezoidal value TN(f) of Composite trapezoidal rule satisfies TN(f)=∫abf(x) dx+∑r=1pB2rh2r(2r)!(f(2r−1)(b)−f(2r−1)(a))−h2p(2p)!∑i=0N−1∫xixi+1B2p(ui(x))f(2p)(x) dx. In particular, with M2p:=max⁡0≤u≤1∣B2p(u)∣, the absolute value of the final remainder is at most (b−a)M2ph2p∥f(2p)∥∞/(2p)!.

Facts & Assumptions

Given: A nondegenerate interval [a,b], integers N,p≥1, the uniform mesh, and f∈C2p([a,b]).

[A1]

The parameters satisfy a<b, N≥1, p≥1, h=(b−a)/N>0, and f has continuous derivatives through order 2p on [a,b].

[F1]

Composite trapezoidal rule: TN(f)=h(f(a)/2+∑i=1N−1f(xi)+f(b)/2).

[F2]

Bernoulli polynomials by a normalized coefficient recursion and Derivative, endpoint, parity, and boundedness identities for Bernoulli polynomials: B1(u)=u−12, Bm′=mBm−1, Bm(1)=Bm(0) for m≥2, and Bm(0)=0 for odd m≥3.

[F3]

Higher derivatives and the classes Ck and C∞: C2p([a,b]) means the derivatives through order 2p are continuous, with one-sided endpoint derivatives.

[F6]

The factorial n! and the falling factorial nk‾, defined by recursion in N: (k)! is the positive product of the first k positive integer real images.

Proof

1.1givenA1F1F2F5F6algebra

On a panel [xi,xi+1], put u=(x−xi)/h. Integration by parts using B1(u)=u−12 gives h∫xixi+1B1(u)f′(x) dx=h2(f(xi)+f(xi+1))−∫xixi+1f(x) dx. Summing this identity over the N panels and using [F1] and [F5] yields TN(f)−∫abf=J1, where Jk:=hkk!∑i=0N−1∫xixi+1Bk(ui(x))f(k)(x) dx for 1≤k≤2p.

1.2givenA1F2F3F4F5algebra

For 1≤k<2p, differentiate Bk+1(ui(x)) to get (k+1)Bk(ui(x))/h. Integrating by parts on each panel therefore gives Jk=hk+1(k+1)!∑i=0N−1[Bk+1(ui(x))f(k)(x)]xixi+1−Jk+1. Because Bk+1(1)=Bk+1(0) by [F2], the interior endpoint terms telescope, leaving hk+1Bk+1(k+1)!(f(k)(b)−f(k)(a)) as the boundary term.

2.1givenstep 1.2F2algebra

Write bq:=Bqhqq!(f(q−1)(b)−f(q−1)(a)) for q≥2. If k+1 is odd and at least 3, [F2] makes that boundary term zero, so step 1.2 gives Jk=−Jk+1. If k+1=2r is even, step 1.2 gives J2r−1=b2r−J2r. For each r<p, the odd-index recurrence at k=2r gives J2r=−J2r+1; hence J2r−1=b2r+J2r+1. Finite induction on r=1,…,p−1 (The principle of mathematical induction) iterates these identities; followed by the final even-index recurrence, it gives J1=∑r=1pB2rh2r(2r)!(f(2r−1)(b)−f(2r−1)(a))−J2p.

3.1givenA1F2F5F6F7step 1.1step 2.1algebra∎

The polynomial B2p is continuous by [F2], so [F7] ensures M2p=max⁡[0,1]∣B2p∣ exists and is finite. By the definition of J2p and the bound ∣B2p(u)∣≤M2p on each unit cell, ∣J2p∣≤h2pM2p∥f(2p)∥∞∑i(xi+1−xi)/(2p)!=(b−a)M2ph2p∥f(2p)∥∞/(2p)!. Substituting step 2.1 into step 1.1 proves the stated formula and remainder bound.

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