Euler–Maclaurin formula


In mathematics, the Euler–Maclaurin formula is a formula for the difference between an integral and a closely related sum. It can be used to approximate integrals by finite sums, or conversely to evaluate finite sums and infinite series using integrals and the machinery of calculus. For example, many asymptotic expansions are derived from the formula, and Faulhaber's formula for the sum of powers is an immediate consequence.
The formula was discovered independently by Leonhard Euler and Colin Maclaurin around 1735. Euler needed it to compute slowly converging infinite series while Maclaurin used it to calculate integrals. It was later generalized to Darboux's formula.

The formula

If and are natural numbers and is a real or complex valued continuous function for real numbers in the interval, then the integral
can be approximated by the sum
. The Euler–Maclaurin formula provides expressions for the difference between the sum and the integral in terms of the higher derivatives evaluated at the endpoints of the interval, that is to say when and.
Explicitly, for a positive integer and a function that is times continuously differentiable on the interval, we have
where is the th Bernoulli number and is an error term which depends on,,, and and is usually small for suitable values of.
The formula is often written with the subscript taking only even values, since the odd Bernoulli numbers are zero except for. In this case we have
or alternatively

The remainder term

The remainder term arises because the integral is usually not exactly equal to the sum. The formula may be derived by applying repeated integration by parts to successive intervals for. The boundary terms in these integrations lead to the main terms of the formula, and the leftover integrals form the remainder term.
The remainder term has an exact expression in terms of the periodized Bernoulli functions. The Bernoulli polynomials may be defined recursively by and, for,
The periodized Bernoulli functions are defined as
where denotes the largest integer less than or equal to .
With this notation, the remainder term equals
When, it can be shown that
where denotes the Riemann zeta function; one approach to prove this inequality is to obtain the Fourier series for the polynomials. The bound is achieved for even when is zero. The term may be omitted for odd but the proof in this case is more complex. Using this inequality, the size of the remainder term can be estimated as

Low-order cases

The Bernoulli numbers from to are Therefore the low-order cases of the Euler-Maclaurin formula are:

Applications

The Basel problem

The Basel problem is to determine the sum
Euler computed this sum to 20 decimal places with only a few terms of the Euler–Maclaurin formula in 1735. This probably convinced him that the sum equals, which he proved in the same year.

Sums involving a polynomial

If is a polynomial and is big enough, then the remainder term vanishes. For instance, if, we can choose to obtain, after simplification,

Approximation of integrals

The formula provides a means of approximating a finite integral. Let be the endpoints of the interval of integration. Fix, the number of points to use in the approximation, and denote the corresponding step size by. Set, so that and. Then:
This may be viewed as an extension of the trapezoid rule by the inclusion of correction terms. Note that this asymptotic expansion is usually not convergent; there is some, depending upon and, such that the terms past order increase rapidly. Thus, the remainder term generally demands close attention.
The Euler–Maclaurin formula is also used for detailed error analysis in numerical quadrature. It explains the superior performance of the trapezoidal rule on smooth periodic functions and is used in certain extrapolation methods. Clenshaw–Curtis quadrature is essentially a change of variables to cast an arbitrary integral in terms of integrals of periodic functions where the Euler–Maclaurin approach is very accurate. This technique is known as a periodizing transformation.

Asymptotic expansion of sums

In the context of computing asymptotic expansions of sums and series, usually the most useful form of the Euler–Maclaurin formula is
where and are integers. Often the expansion remains valid even after taking the limits or or both. In many cases the integral on the right-hand side can be evaluated in closed form in terms of elementary functions even though the sum on the left-hand side cannot. Then all the terms in the asymptotic series can be expressed in terms of elementary functions. For example,
Here the left-hand side is equal to, namely the first-order polygamma function defined by ; the gamma function is equal to if is a positive integer. This results in an asymptotic expansion for. That expansion, in turn, serves as the starting point for one of the derivations of precise error estimates for Stirling's approximation of the factorial function.

Examples

If is an integer greater than 1 we have:
Collecting the constants into a value of the Riemann zeta function, we can write an asymptotic expansion:
For equal to 2 this simplifies to
or
When, the corresponding technique gives an asymptotic expansion for the harmonic numbers:
where is the Euler–Mascheroni constant.

Proofs

Derivation by mathematical induction

We outline the argument given in Apostol.
The Bernoulli polynomials and the periodic Bernoulli functions for were introduced above.
The first several Bernoulli polynomials are
The values are the Bernoulli numbers. Notice that for we have
and for,
The functions agree with the Bernoulli polynomials on the interval and are periodic with period 1. Furthermore, except when, they are also continuous. Thus,
Let be an integer, and consider the integral
where
Integrating by parts, we get
Using,, and summing the above from to, we get
Adding − f)/2 to both sides and rearranging, we have
This is the case of the summation formula. To continue the induction, we apply integration by parts to the error term:
where
The result of integrating by parts is
Summing from to and substituting this for the lower order error term results in the case of the formula,
This process can be iterated. In this way we get a proof of the Euler–Maclaurin summation formula which can be formalized by mathematical induction, in which the induction step relies on integration by parts and on identities for periodic Bernoulli functions.