200LX, 17b, 19b - Solver Problem?

[Peter A. Gebhardt]

Dear fellow forists,

just stumbled over an issue with the solver which did manifest in "jumps" of a solution value after a discrete change of the variable T in the following equation.

The equation resembles the built-in routine called "STAFF" in the German finance menu of 17bx and 19bx respectively (only international versions of the calculators mentioned do have it in their ROMs).

The routine is from the German Manual of the 17b (the one in the 17bII+ manual contains errors).

T=17:    

T=18:    

T=19:    

Graphing the behavior of below equation for results of T on the HP Emulator results in:

   

Code:


0*(#YRS+NOM%+PV+PMT+FV+T+#P+#C)

+0*
( L(I,NOM%/(100*#C))
 +L(ADJ1,#P/#C+G(I)*((#P/#C+1)/2-T*#P/360))
 +L(N,RND(#YRS*360:8)*#C/360)
 +L(#PP,1-T*#P/360+FP(G(N))*#P/#C)  
 +L(ADJ2,IP(G(#PP))
    +G(I)*IP(G(#PP))
    *(FP(G(#PP))-1+G(#PP))/(2*#P)*#C
  )
)
=
FV
+( PMT*G(ADJ1)*USFV(G(I)*100:IP(G(N)))
  +PV*(1+G(I))^IP(G(N))
 )*(1+G(I)*FP(G(N)))
+PMT*G(ADJ2)

The example I've tested with is here (from p. 295 of the German 17bII+ manual):

   

Any ideas what goes wrong here?
TIA.
Peter