**ACTION:**- Invert a linear model of the type produced by the
sla_FITXY routine.
**CALL:**`CALL sla_INVF (FWDS,BKWDS,J)`

**GIVEN:**-

*FWDS***D(6)**model coefficients

**RETURNED:**-

*BKWDS***D(6)**inverse model *J***I**status: 0 = OK, -1 = no inverse

**NOTES:**- 1.
- The models relate two sets of coordinates as follows.
Naming the six elements of FWDS
*a*,*b*,*c*,*d*,*e*&*f*, where two sets of coordinates [*x*,_{1}*y*] and are related thus:_{1}

The present routine generates a new set of coefficients*x*=_{2}*a*+*bx*+_{1}*cy*_{1}

*y*=_{2}*d*+*ex*+_{1}*fy*_{1}*p*,*q*,*r*,*s*,*t*&*u*(the array BKWDS) such that:*x*=_{1}*p*+*qx*+_{2}*ry*_{2}

*y*=_{1}*s*+*tx*+_{2}*uy*_{2} - 2.
- Two successive calls to this routine will deliver a set of coefficients equal to the starting values.
- 3.
- To comply with the ANSI Fortran 77 standard, FWDS and BKWDS must
**not**be the same array. The routine is, in fact, coded so as to work properly on the VAX and many other systems even if this rule is violated, something that is**not**, however, recommended. - 4.
- See also sla_FITXY, sla_PXY, sla_XY2XY, sla_DCMPF.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk