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Schlömilch's Series


A Fourier series-like expansion of a twice continuously differentiable function

 f(x)=1/2a_0+sum_(n=1)^inftya_nJ_0(nx)
(1)

for 0<x<pi, where J_0(x) is a zeroth order Bessel function of the first kind. The coefficients are then given by

a_0=2f(0)+2/piint_0^piint_0^(pi/2)uf^'(usinphi)dphidu
(2)
a_n=2/piint_0^piint_0^(pi/2)uf^'(usinphi)cos(nu)dphidu
(3)

(Gradshteyn and Ryzhik 2000, p. 926), where f^'(x)=df/dx and care should be taken to avoid the two typos of Iyanaga and Kawada (1980) and Itô (1986).

SchloemilchSeries

As an example, consider f(x)=x, which has f^'(x)=1 and therefore

a_0=0+2/piint_0^piint_0^(pi/2)udphidu
(4)
=1/2pi^2
(5)
a_n=2/piint_0^piint_0^(pi/2)ucos(nu)dphidu
(6)
=(-1+(-1)^n)/(n^2)
(7)
={0 for n even; -2/(n^2) for n odd,
(8)

so

 x=1/4pi^2-2sum_(n=1,3,...)^infty(J_0(nx))/(n^2)
(9)

(Whittaker and Watson 1990, p. 378; Gradshteyn and Ryzhik 2000, p. 926). This is illustrated above with 1 (red), 2 (green), 3 (blue), and 4 terms (violet) included.

Similarly, for -pi<x<pi,

 x^2=(2pi^2)/3+8sum_(n=1)^infty((-1)^n)/(n^2)J_0(nx).
(10)

See also

Bessel Function of the First Kind, Fourier-Bessel Series, Fourier Series

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References

Chapman. Quart. J. 43, 34-37, 1912.Gradshteyn, I. S. and Ryzhik, I. M. "The Series sumA_kJ_0(kx)." Tables of Integrals, Series, and Products, 6th ed. San Diego, CA: Academic Press, p. 926, 2000.Itô, K. (Ed.). Encyclopedic Dictionary of Mathematics, 2nd ed., Vol. 2. Cambridge, MA: MIT Press, p. 1803, 1986.Iyanaga, S. and Kawada, Y. (Eds.). Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press, p. 1473, 1980.Schlömilch. Z. für Math. Phys. 3, 137-165, 1857.Whittaker, E. T. and Watson, G. N. "Schlömilch's Expansion of an Arbitrary Function in a Series of Bessel Coefficients of Order Zero." §17.82 in A Course in Modern Analysis, 4th ed. Cambridge, England: Cambridge University Press, pp. 377-378, 1990.

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Schlömilch's Series

Cite this as:

Weisstein, Eric W. "Schlömilch's Series." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/SchloemilchsSeries.html

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