## Often (most of the time??) the relevant system of equations is

__not linear in the unknowns__.## Then, cannot decompose as

*A***x**=**b**. Oh well.## Instead write as:

*f*(*x*) = 0 function of one variable (1-D)**f**(**x**) =**0****x**= (*x*1,*x*2,...,*xn*),**f**= (*f*1,*f*2,...,*fn*) (*n*-D)

## Not guaranteed to have

__any__real solutions, but generally do for astrophysical problems.