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CHAPTER I.

INTRODUCTORY.

DEFINITION OF SPHERICAL HARMONICS.

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1. IF V be the potential of an attracting mass, at any point x, y, z, not forming a part of the mass itself, it is known that V must satisfy the differential equation

d'y d'V dV
dict dy
+ 0 .....

.(1),

dz2 or, as we shall write it for shortness, V*V = 0.

The general solution of this equation cannot be obtained in finite terms. We can, however, determine an expression which we shall call Vi, an homogeneous function of x, y, z of the degree i, i being any positive integer, which will satisfy, the equation; and we may prove that to every such solution V, there corresponds another, of the degree - (i+1),

V. expressed by tti, where r*=x*+y*+z".

For the equation (1) when transformed to polar co-ordinates by writing x=r sin 0 cos 0, y=r sin sin , z=r cos 0, becomes do (V) 1 d

dV l
+
sin e
+

= 0...(2). dr2 sin Ꮎ ddo

sino do And since V satisfies this equation, and is an homogeneous function of the degree i, Vi must satisfy the equation 1 d

1 d'V; ii + 1) Vit

e

0, do

1

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(si o

2

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F. H.

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