■シンク関数の積分実験(その2)

,丹野の定理を拡張すると,正の数aと整数n≧2に対して

 (定理)

  ∫(0,∞)sin^n(ax)/x^ndx=a^(n-1)π/2-a^(n-1)π/(m-1)!2^(n-1)Σ(-1)^(r-1)(n-1,r-1)(n-2r)^(n-1)

  (n-1,r-1)は2項係数n-1Cr-1,r=0~[(n-1)/2],[]はガウス記号

が成り立ちます.

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 この定理を用いると

  ∫(0,∞)sin(ax)/xdx=π/2

  ∫(0,∞)sin^2(ax)/x^2dx=aπ/2

  ∫(0,∞)sin^3(ax)/x^3dx=3a^2π/8

  ∫(0,∞)sin^4(ax)/x^4dx=a^3π/3

  ∫(0,∞)sin^5(ax)/x^5dx=115a^4π/384

  ∫(0,∞)sin^6(ax)/x^6dx=11a^5π/40

が求められます.

 

 ちなみに,詳しい公式集,

  "Table of Integrals,Series and Products"

  I.S.Gradshteyn and I.M.Ryzhik,6th Edition,Academic Press

p456-457でも,この形の積分に関しては,

  ∫(0,∞)sin^6(ax)/x^6dx=11a^5π/40

までしか載っていません.

 

 しかし,丹野の定理を使えば,ずっと先まで求めることができるというわけです.

  ∫(0,∞)sin^7(ax)/x^7dx=5887a^6π/23040

  ∫(0,∞)sin^8(ax)/x^8dx=151a^7π/630

  ∫(0,∞)sin^9(ax)/x^9dx=259732a^8π/1146880

  ∫(0,∞)sin^10(ax)/x^10=15619a^9π/72576

  ∫(0,∞)sin^11(ax)/x^11dx=381773117a^10π/1857945600

  ∫(0,∞)sin^12(ax)/x^12dx=655177a^11π/3326400

  ∫(0,∞)sin^13(ax)/x^13dx=20646903199a^12π/108999475200

  ∫(0,∞)sin^14(ax)/x^14dx=27085381a^13π/148262400

  ∫(0,∞)sin^15(ax)/x^15dx=467168310097a^14π/2645053931520

 

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