HP 35s Scientific Calculator - Manual (Page 245)

Solving and Integrating Programs 15-11
Restrictions on Solving and Integrating
The SOLVE variable and ∫ FN d variable instructions cannot call a routine that
contains another SOLVE or ∫ FN instruction. That is, neither of these instructions can
be used recursively. For example, attempting to calculate a multiple integral will
result in an ∫
∫  error. Also, SOLVE and ∫ FN cannot call a routine that contains
an label instruction; if attempted, a   or ∫   error
will be returned. SOLVE cannot call a routine that contains an ∫ FN instruction
(produces a ∫  error), just as ∫ FN cannot call a routine that contains a
SOLVE instruction (produces an ∫  error).
The SOLVE variable and ∫ FN d variable instructions in a program use one of the 20
pending subroutine returns in the calculator. (Refer to "Nested Subroutines" in
chapter 14.)
  
  
  
  
 ∫   
Recalls lower limit of integration.
Recalls upper limit of integration. (X = D.)
Specifies the function.
Integrates the normal function using the dummy variable D.