title: Nullcline
text: # Nullcline
  
  In mathematical analysis, nullclines, sometimes called zero-growth isoclines, are encountered in a system of ordinary differential equations
  
      $x_{1}'=f_{1}(x_{1},\ldots ,x_{n})$
      $x_{2}'=f_{2}(x_{1},\ldots ,x_{n})$
         $\vdots$
      $x_{n}'=f_{n}(x_{1},\ldots ,x_{n})$
  
  where $x'$ here represents a derivative of $x$ with respect to another parameter, such as time $t$. The $j$'th nullcline is the geometric shape for which $x_{j}'=0$. The equilibrium points of the system are located where all of the nullclines intersect. In a two-dimensional linear system, the nullclines can be represented by two lines on a two-dimensional plot; in a general two-dimensional system they are arbitrary curves.
  
  ## History
  
  The definition, though with the name ’directivity curve’, was used in a 1967 article by Endre Simonyi. This article also defined 'directivity vector' as $\mathbf {w} =\mathrm {sign} (P)\mathbf {i} +\mathrm {sign} (Q)\mathbf {j}$, where P and Q are the dx/dt and dy/dt differential equations, and i and j are the x and y direction unit vectors.
  
  Simonyi developed a new stability test method from these new definitions, and with it he studied differential equations. This method, beyond the usual stability examinations, provided semi-quantitative results.
  
  