释义 |
Helmholtz Differential Equation--Spherical SurfaceOn the surface of a Sphere, attempt Separation of Variables in Spherical Coordinates by writing
 | (1) |
then the Helmholtz Differential Equation becomes
 | (2) |
Dividing both sides by ,
 | (3) |
which can now be separated by writing
 | (4) |
The solution to this equation must be periodic, so must be an Integer. The solution may then be defined either asa Complex function
 | (5) |
for , ..., , or as a sum of Real sine and cosine functions
 | (6) |
for , ..., . Plugging (4) into (3) gives
 | (7) |
 | (8) |
which is the Legendre Differential Equation for with
 | (9) |
giving
 | (10) |
 | (11) |
Solutions are therefore Legendre Polynomials with a Complex index. The general Complex solution is then
 | (12) |
and the general Real solution is
 | (13) |
Note that these solutions depend on only a single variable . However, on the surface of a sphere, it is usual to expresssolutions in terms of the Spherical Harmonics derived for the 3-D spherical case, which depend onthe two variables and . |