■正弦積分(その4)
(定理2)
∫(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.
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2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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cm=∫(0,∞)|d/dx{sin(x)/x}^m|dx-1
c2=(e^2-7)/2
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