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732 Chapter 23 Fourier series

Solutions

1 (a) odd (b) neither

(c) odd (d) even

2 (a) even (b) odd (c) even

(d) odd (e) neither

3 (a)0 (b)0

(c) 0 (d) 0

(e) 1 (f) 0

23.4 ORTHOGONALITYRELATIONSANDOTHER

USEFULIDENTITIES

RecallfromChapter16thattwofunctions f (t)andg(t)aresaidtobeorthogonalonthe

intervala t bif

∫ b

a

f(t)g(t)dt =0

Example23.12 Show that the functions cosmωt and cosnωt withm,npositive integers andm ≠n are

orthogonal on the interval − π ω t π ω .

Solution We mustevaluate

∫ π/ω

−π/ω

cosmωt cosnωtdt

Usingthetrigonometricidentity2cosAcosB = cos(A +B) +cos(A −B),wefindthe

integral becomes

1 π/ω

cos(m +n)ωt +cos(m −n)ωtdt

2 −π/ω

= 1 2

[ sin(m +n)ωt

(m +n)ω

]

sin(m −n)ωt π/ω

+

(m −n)ω

−π/ω

= 0

since sin(m ±n)π = 0 for all integersm,n. It was necessary to requirem ≠ n since

otherwise the second quantity inbrackets becomes undefined.

Hence cosmωt and cosnωt are orthogonal on the given interval.

AnumberofotherfunctionsregularlyappearinginworkconnectedwithFourieranalysis

are orthogonal. The main results together with some other useful integral identities are

given inTable 23.1.

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