The following comes from General System Theory by Ludwid von Betalanffy, 1968:
Sets of simultaneous differential equations as a way to "model" or define a system are, if linear, tiresome to solve even in the case of a few variables; if nonlinear, they are unsolvable except in special cases (Table 1.1).
Table 1.1

.  .  .  Linear  .  .  .  .  Nonlinear  . 

Equation Type:  One Equation 
.  Several Equations 
.  Many Equations 
.  One Equation 
Several Equations 
Many Equations 
Algebraic  Trivial  .  Easy  .  Essentially Impossible  .  Very Difficult  Very Difficult  Impossible 
Ordinary Differential  Easy  .  Difficult  .  Essentially Impossible  .  Essentially Impossible  Impossible  Impossible 
.  .  .  x  .  .  .  .  .  . 
Partial Differential  Difficult  .  Essentially Impossible  .  Essentially Impossible  .  Impossible  Impossible  Impossible 
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