Reference:Poly

Higher order polynomial surfaces may be defined by the use of a poly shape.

The syntax is

POLY:
  poly {
    Order, <A1, A2, A3,... An>
    [POLY_MODIFIERS...]
    }
POLY_MODIFIERS:
  sturm | OBJECT_MODIFIER

Poly default values:

sturm : off

Where Order is an integer number from 2 to 35 inclusively that specifies the order of the equation. A1, A2, ...An are float values for the coefficients of the equation. There are n such terms where n = ((Order+1)*(Order+2)*(Order+3))/6.

If additional accuracy is required you can add the sturm object modifier.