text: # Biharmonic equation
  
  In mathematics, the biharmonic equation is a fourth-order partial differential equation which arises in areas of continuum mechanics, including linear elasticity theory and the solution of Stokes flows. It is written as
  
  $\nabla^4\varphi=0$
  
  or
  
  $\nabla^2\nabla^2\varphi=0$
  
  or
  
  $\Delta^2\varphi=0$
  
  where $\nabla^4$ is the fourth power of the del operator and the square of the laplacian operator $\nabla^2$ (or $\Delta$), and it is known as the biharmonic operator or the bilaplacian operator.
  
  For example, in three dimensional cartesian coordinates the biharmonic equation has the form
  
  ${\partial^4 \varphi\over \partial x^4 } + {\partial^4 \varphi\over \partial y^4 } + {\partial^4 \varphi\over \partial z^4 }+ 2{\partial^4 \varphi\over \partial x^2\partial y^2}+ 2{\partial^4 \varphi\over \partial y^2\partial z^2}+ 2{\partial^4 \varphi\over \partial x^2\partial z^2} = 0.$
  
  As another example, in n-dimensional Euclidean space,
  
  $\nabla^4 \left({1\over r}\right)= {3(15-8n+n^2)\over r^5}$
  
  where
  
  $r=\sqrt{x_1^2+x_2^2+\cdots+x_n^2}.$
  
  which, for n=3 and n=5 only, becomes the biharmonic equation.
  
  A solution to the biharmonic equation is called a biharmonic function. Any harmonic function is biharmonic, but the converse is not always true.
  
  In two-dimensional polar coordinates, the biharmonic equation is
  
  $\frac{1}{r} \frac{\partial}{\partial r} \left(r \frac{\partial}{\partial r} \left(\frac{1}{r} \frac{\partial}{\partial r} \left(r \frac{\partial \varphi}{\partial r}\right)\right)\right) + \frac{2}{r^2} \frac{\partial^4 \varphi}{\partial \theta^2 \partial r^2} + \frac{1}{r^4} \frac{\partial^4 \varphi}{\partial \theta^4} - \frac{2}{r^3} \frac{\partial^3 \varphi}{\partial \theta^2 \partial r} + \frac{4}{r^4} \frac{\partial^2 \varphi}{\partial \theta^2} = 0$
  
  which can be solved by separation of variables. The result is the Michell solution.
  
  ## 2 dimensional space
  
  The general solution to the 2 dimensional case is
  
  $x v(x,y) - y u(x,y) + w(x,y)$
  
  where $u(x,y)$, $v(x,y)$ and $w(x,y)$ are harmonic functions and $v(x,y)$ is a harmonic conjugate of $u(x,y)$.
  
  Just as harmonic functions in 2 variables are closely related to complex analytic functions, so are biharmonic functions in 2 variables. The general form of a biharmonic function in 2 variables can also be written as
  
  $\operatorname{Im}(\bar{z}f(z) + g(z))$
  
  where $f(z)$ and $g(z)$ are analytic functions.