Bernoulli polynomials

The Bernoulli polynomials B_n can be defined by a generating function. They also admit a variety of derived representations.

An explicit formula for the Bernoulli polynomials is given by

${\displaystyle B_{m}(x)=\sum _{n=0}^{m}{\frac {1}{n+1}}\sum _{k=0}^{n}(-1)^{k}{n \choose k}(x+k)^{m}.}$

That is similar to the series expression for the Hurwitz zeta function in the complex plane. Indeed, there is the relationship

${\displaystyle B_{n}(x)=-n\zeta (1-n,x)}$

where ζ(s, q) is the Hurwitz zeta function. The latter generalizes the Bernoulli polynomials, allowing for non-integer values of n.

The inner sum may be understood to be the nth forward difference of x^m; that is,

${\displaystyle \Delta ^{n}x^{m}=\sum _{k=0}^{n}(-1)^{n-k}{n \choose k}(x+k)^{m}}$

where Δ is the forward difference operator. Thus, one may write

${\displaystyle B_{m}(x)=\sum _{n=0}^{m}{\frac {(-1)^{n}}{n+1}}\,\Delta ^{n}x^{m}.}$

This formula may be derived from an identity appearing above as follows. Since the forward difference operator Δ equals

${\displaystyle \Delta =e^{D}-1}$

where D is differentiation with respect to x, we have, from the Mercator series,

${\displaystyle {D \over e^{D}-1}={\log(\Delta +1) \over \Delta }=\sum _{n=0}^{\infty }{(-\Delta )^{n} \over n+1}.}$

As long as this operates on an mth-degree polynomial such as x^m, one may let n go from 0 only up to m.

An integral representation for the Bernoulli polynomials is given by the Nörlund–Rice integral, which follows from the expression as a finite difference.

An explicit formula for the Euler polynomials is given by

${\displaystyle E_{m}(x)=\sum _{n=0}^{m}{\frac {1}{2^{n}}}\sum _{k=0}^{n}(-1)^{k}{n \choose k}(x+k)^{m}\,.}$

The above follows analogously, using the fact that

${\displaystyle {\frac {2}{e^{D}+1}}={\frac {1}{1+\Delta /2}}=\sum _{n=0}^{\infty }{\Bigl (}-{\frac {\Delta }{2}}{\Bigr )}^{n}.}$

Using either the above integral representation of ${\displaystyle x^{n}}$ or the identity ${\displaystyle B_{n}(x+1)-B_{n}(x)=nx^{n-1}}$, we have

${\displaystyle \sum _{k=0}^{x}k^{p}=\int _{0}^{x+1}B_{p}(t)\,dt={\frac {B_{p+1}(x+1)-B_{p+1}}{p+1}}}$

(assuming 0⁰ = 1). See Faulhaber's formula for more on this.

The Bernoulli numbers are given by ${\displaystyle \textstyle B_{n}=B_{n}(0).}$

This definition gives ${\displaystyle \textstyle \zeta (-n)={\frac {(-1)^{n}}{n+1}}B_{n+1}}$ for ${\displaystyle \textstyle n=0,1,2,\ldots }$.

An alternate convention defines the Bernoulli numbers as ${\displaystyle \textstyle B_{n}=B_{n}(1).}$

The two conventions differ only for ${\displaystyle n=1}$ since ${\displaystyle B_{1}(1)={\tfrac {1}{2}}=-B_{1}(0)}$.

The Euler numbers are given by ${\displaystyle E_{n}=2^{n}E_{n}({\tfrac {1}{2}}).}$

The first few Bernoulli polynomials are:

${\displaystyle {\begin{aligned}B_{0}(x)&=1\\[8pt]B_{1}(x)&=x-{\frac {1}{2}}\\[8pt]B_{2}(x)&=x^{2}-x+{\frac {1}{6}}\\[8pt]B_{3}(x)&=x^{3}-{\frac {3}{2}}x^{2}+{\frac {1}{2}}x\\[8pt]B_{4}(x)&=x^{4}-2x^{3}+x^{2}-{\frac {1}{30}}\\[8pt]B_{5}(x)&=x^{5}-{\frac {5}{2}}x^{4}+{\frac {5}{3}}x^{3}-{\frac {1}{6}}x\\[8pt]B_{6}(x)&=x^{6}-3x^{5}+{\frac {5}{2}}x^{4}-{\frac {1}{2}}x^{2}+{\frac {1}{42}}.\end{aligned}}}$

The first few Euler polynomials are:

${\displaystyle {\begin{aligned}E_{0}(x)&=1\\[8pt]E_{1}(x)&=x-{\frac {1}{2}}\\[8pt]E_{2}(x)&=x^{2}-x\\[8pt]E_{3}(x)&=x^{3}-{\frac {3}{2}}x^{2}+{\frac {1}{4}}\\[8pt]E_{4}(x)&=x^{4}-2x^{3}+x\\[8pt]E_{5}(x)&=x^{5}-{\frac {5}{2}}x^{4}+{\frac {5}{2}}x^{2}-{\frac {1}{2}}\\[8pt]E_{6}(x)&=x^{6}-3x^{5}+5x^{3}-3x.\end{aligned}}}$

At higher n, the amount of variation in B_n(x) between x = 0 and x = 1 gets large. For instance,

${\displaystyle B_{16}(x)=x^{16}-8x^{15}+20x^{14}-{\frac {182}{3}}x^{12}+{\frac {572}{3}}x^{10}-429x^{8}+{\frac {1820}{3}}x^{6}-{\frac {1382}{3}}x^{4}+140x^{2}-{\frac {3617}{510}}}$

which shows that the value at x = 0 (and at x = 1) is −3617/510 ≈ −7.09, while at x = 1/2, the value is 118518239/3342336 ≈ +7.09. D.H. Lehmer[1] showed that the maximum value of B_n(x) between 0 and 1 obeys

${\displaystyle M_{n}<{\frac {2n!}{(2\pi )^{n}}}}$

unless n is 2 modulo 4, in which case

${\displaystyle M_{n}={\frac {2\zeta (n)n!}{(2\pi )^{n}}}}$

(where ${\displaystyle \zeta (x)}$ is the Riemann zeta function), while the minimum obeys

${\displaystyle m_{n}>{\frac {-2n!}{(2\pi )^{n}}}}$

unless n is 0 modulo 4, in which case

${\displaystyle m_{n}={\frac {-2\zeta (n)n!}{(2\pi )^{n}}}.}$

These limits are quite close to the actual maximum and minimum, and Lehmer gives more accurate limits as well.

The Bernoulli and Euler polynomials obey many relations from umbral calculus:

${\displaystyle \Delta B_{n}(x)=B_{n}(x+1)-B_{n}(x)=nx^{n-1},}$

${\displaystyle \Delta E_{n}(x)=E_{n}(x+1)-E_{n}(x)=2(x^{n}-E_{n}(x)).}$

(Δ is the forward difference operator). Also,

${\displaystyle E_{n}(x+1)+E_{n}(x)=2x^{n}.}$

These polynomial sequences are Appell sequences:

${\displaystyle B_{n}'(x)=nB_{n-1}(x),}$

${\displaystyle E_{n}'(x)=nE_{n-1}(x).}$

The Fourier series of the Bernoulli polynomials is also a Dirichlet series, given by the expansion

${\displaystyle B_{n}(x)=-{\frac {n!}{(2\pi i)^{n}}}\sum _{k\not =0}{\frac {e^{2\pi ikx}}{k^{n}}}=-2n!\sum _{k=1}^{\infty }{\frac {\cos \left(2k\pi x-{\frac {n\pi }{2}}\right)}{(2k\pi )^{n}}}.}$

Note the simple large n limit to suitably scaled trigonometric functions.

This is a special case of the analogous form for the Hurwitz zeta function

${\displaystyle B_{n}(x)=-\Gamma (n+1)\sum _{k=1}^{\infty }{\frac {\exp(2\pi ikx)+e^{i\pi n}\exp(2\pi ik(1-x))}{(2\pi ik)^{n}}}.}$

This expansion is valid only for 0 ≤ x ≤ 1 when n ≥ 2 and is valid for 0 < x < 1 when n = 1.

The Fourier series of the Euler polynomials may also be calculated. Defining the functions

${\displaystyle C_{\nu }(x)=\sum _{k=0}^{\infty }{\frac {\cos((2k+1)\pi x)}{(2k+1)^{\nu }}}}$

and

${\displaystyle S_{\nu }(x)=\sum _{k=0}^{\infty }{\frac {\sin((2k+1)\pi x)}{(2k+1)^{\nu }}}}$

for ${\displaystyle \nu >1}$, the Euler polynomial has the Fourier series

${\displaystyle C_{2n}(x)={\frac {(-1)^{n}}{4(2n-1)!}}\pi ^{2n}E_{2n-1}(x)}$

and

${\displaystyle S_{2n+1}(x)={\frac {(-1)^{n}}{4(2n)!}}\pi ^{2n+1}E_{2n}(x).}$

Note that the ${\displaystyle C_{\nu }}$ and ${\displaystyle S_{\nu }}$ are odd and even, respectively:

${\displaystyle C_{\nu }(x)=-C_{\nu }(1-x)}$

and

${\displaystyle S_{\nu }(x)=S_{\nu }(1-x).}$

They are related to the Legendre chi function ${\displaystyle \chi _{\nu }}$ as

${\displaystyle C_{\nu }(x)=\operatorname {Re} \chi _{\nu }(e^{ix})}$

and

${\displaystyle S_{\nu }(x)=\operatorname {Im} \chi _{\nu }(e^{ix}).}$

The Bernoulli and Euler polynomials may be inverted to express the monomial in terms of the polynomials.

Specifically, evidently from the above section on integral operators, it follows that

${\displaystyle x^{n}={\frac {1}{n+1}}\sum _{k=0}^{n}{n+1 \choose k}B_{k}(x)}$

and

${\displaystyle x^{n}=E_{n}(x)+{\frac {1}{2}}\sum _{k=0}^{n-1}{n \choose k}E_{k}(x).}$

The Bernoulli polynomials may be expanded in terms of the falling factorial ${\displaystyle (x)_{k}}$ as

${\displaystyle B_{n+1}(x)=B_{n+1}+\sum _{k=0}^{n}{\frac {n+1}{k+1}}\left\{{\begin{matrix}n\\k\end{matrix}}\right\}(x)_{k+1}}$

where ${\displaystyle B_{n}=B_{n}(0)}$ and

${\displaystyle \left\{{\begin{matrix}n\\k\end{matrix}}\right\}=S(n,k)}$

denotes the Stirling number of the second kind. The above may be inverted to express the falling factorial in terms of the Bernoulli polynomials:

${\displaystyle (x)_{n+1}=\sum _{k=0}^{n}{\frac {n+1}{k+1}}\left[{\begin{matrix}n\\k\end{matrix}}\right]\left(B_{k+1}(x)-B_{k+1}\right)}$

where

${\displaystyle \left[{\begin{matrix}n\\k\end{matrix}}\right]=s(n,k)}$

denotes the Stirling number of the first kind.

The multiplication theorems were given by Joseph Ludwig Raabe in 1851:

For a natural number m≥1,

${\displaystyle B_{n}(mx)=m^{n-1}\sum _{k=0}^{m-1}B_{n}\left(x+{\frac {k}{m}}\right)}$

${\displaystyle E_{n}(mx)=m^{n}\sum _{k=0}^{m-1}(-1)^{k}E_{n}\left(x+{\frac {k}{m}}\right)\quad {\mbox{ for }}m=1,3,\dots }$

${\displaystyle E_{n}(mx)={\frac {-2}{n+1}}m^{n}\sum _{k=0}^{m-1}(-1)^{k}B_{n+1}\left(x+{\frac {k}{m}}\right)\quad {\mbox{ for }}m=2,4,\dots }$

Two definite integrals relating the Bernoulli and Euler polynomials to the Bernoulli and Euler numbers are:

• ${\displaystyle \int _{0}^{1}B_{n}(t)B_{m}(t)\,dt=(-1)^{n-1}{\frac {m!n!}{(m+n)!}}B_{n+m}\quad {\text{for }}m,n\geq 1}$
• ${\displaystyle \int _{0}^{1}E_{n}(t)E_{m}(t)\,dt=(-1)^{n}4(2^{m+n+2}-1){\frac {m!n!}{(m+n+2)!}}B_{n+m+2}}$

A periodic Bernoulli polynomial P_n(x) is a Bernoulli polynomial evaluated at the fractional part of the argument x. These functions are used to provide the remainder term in the Euler–Maclaurin formula relating sums to integrals. The first polynomial is a sawtooth function.

Strictly these functions are not polynomials at all and more properly should be termed the periodic Bernoulli functions, and P₀(x) is not even a function, being the derivative of a sawtooth and so a Dirac comb.

The following properties are of interest, valid for all ${\displaystyle x}$:

${\displaystyle {\begin{aligned}&P_{k}(x){\text{ is continuous for all }}k>1\\[5pt]&P_{k}'(x){\text{ exists and is continuous for }}k>2\\[5pt]&P'_{k}(x)=kP_{k-1}(x),k>2\end{aligned}}}$

• Bernoulli numbers
• Bernoulli polynomials of the second kind
• Stirling polynomial

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• Apostol, Tom M. (1976), Introduction to analytic number theory, Undergraduate Texts in Mathematics, New York-Heidelberg: Springer-Verlag, ISBN 978-0-387-90163-3, MR 0434929, Zbl 0335.10001 (See chapter 12.11)
• Dilcher, K. (2010), "Bernoulli and Euler Polynomials", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248
• Cvijović, Djurdje; Klinowski, Jacek (1995). "New formulae for the Bernoulli and Euler polynomials at rational arguments". Proceedings of the American Mathematical Society. 123: 1527–1535. doi:10.2307/2161144.
• Guillera, Jesus; Sondow, Jonathan (2008). "Double integrals and infinite products for some classical constants via analytic continuations of Lerch's transcendent". The Ramanujan Journal. 16 (3): 247–270. arXiv:math.NT/0506319. doi:10.1007/s11139-007-9102-0. (Reviews relationship to the Hurwitz zeta function and Lerch transcendent.)
• Hugh L. Montgomery; Robert C. Vaughan (2007). Multiplicative number theory I. Classical theory. Cambridge tracts in advanced mathematics. 97. Cambridge: Cambridge Univ. Press. pp. 495–519. ISBN 0-521-84903-9.

• A list of integral identities involving Bernoulli polynomials from NIST