# Laplace-like operator

Help me please to apply a Laplace-like operator:$\Delta f:= \frac{\partial^2 f}{\partial r^2} + \frac{\partial^2 f}{\partial z^2} + {1\over r}\,\frac{\partial f}{\partial r} - {f\over r^2}$ on the expression: $f:=\frac{r}{a}\rho^{-\alpha}\sin (\alpha\phi)$.

when $\rho=\sqrt{(r-a)^{2}+z^{2}}$

Thanks a lot!

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Are $\alpha$ and $\phi$ constant? In any case, write $f$ as a function of $r$ and $z$, then compute the derivatives which appear in the expression of $\Delta$. – Davide Giraudo Oct 5 '12 at 16:49
Thank you ...!! – Mushka Oct 7 '12 at 19:30
Did you manage to solve the problem? – Davide Giraudo Oct 7 '12 at 19:32

You can use MATLAB Program :

   syms r a z alpha phi
rho=sqrt((r-a)^2+z^2);
f=(r/a)*rho^(-alpha)*sin(alpha*phi);
delta_f=diff(f,r,2)+diff(f,z,2)+(1/r)*diff(f,r)-f/(r^2);
delta_f=simple(delta_f);
pretty(delta_f)


$$\Delta f:= \frac{\partial^2 f}{\partial r^2} + \frac{\partial^2 f}{\partial z^2} + {1\over r}\,\frac{\partial f}{\partial r} - {f\over r^2}$$ $$\Delta f := \frac{\alpha \space sin (\alpha \phi) (3a -3r+r \alpha)}{\alpha (a^2-2ra+r^2+z^2)^{\frac{\alpha}{2}+1}}$$

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