Spherical harmonic expansion for a function

[rajesh7796]

Homework Statement

The spherical harmonic expansion for exp^ik(r-a)/(r-a), where the quantities in brackets are in absolute values and r and a are vectors , is well known and found in many textbooks.

Homework Equations

any hints on finding the spherical harmonic expansion for (r-a)^2 (exp^ik(r-a)/(r-a)),

The Attempt at a Solution

 

[turin]

Differentiation (w.r.t. k).

BTW, doesn't one of the |r-a|'s out front cancel the |r-a| in the denominator?