(maxima.info)Functions and Variables for stirling

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84.1 Functions and Variables for stirling
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 -- Function: stirling
          stirling (<z>,<n>)
          stirling (<z>,<n>,<pred>)

     Replace 'gamma(x)' with the O(1/x^(2n-1)) Stirling formula.  when
     <n> isn't a nonnegative integer, signal an error.  With the
     optional third argument 'pred', the Stirling formula is applied
     only when 'pred' is true.

     Reference: Abramowitz & Stegun, " Handbook of mathematical
     functions", 6.1.40.

     Examples:
          (%i1) load ("stirling")$

          (%i2) stirling(gamma(%alpha+x)/gamma(x),1);
                 1/2 - x             x + %alpha - 1/2
          (%o2) x        (x + %alpha)
                                             1           1
                                      --------------- - ---- - %alpha
                                      12 (x + %alpha)   12 x
                                    %e
          (%i3) taylor(%,x,inf,1);
                              %alpha       2    %alpha
                    %alpha   x       %alpha  - x       %alpha
          (%o3)/T/ x       + -------------------------------- + . . .
                                           2 x
          (%i4) map('factor,%);
                                                 %alpha - 1
                   %alpha   (%alpha - 1) %alpha x
          (%o4)   x       + -------------------------------
                                            2

     The function 'stirling' knows the difference between the variable
     'gamma' and the function gamma:

          (%i5) stirling(gamma + gamma(x),0);
                                              x - 1/2   - x
          (%o5)    gamma + sqrt(2) sqrt(%pi) x        %e
          (%i6) stirling(gamma(y) + gamma(x),0);
                                   y - 1/2   - y
          (%o6) sqrt(2) sqrt(%pi) y        %e
                                                        x - 1/2   - x
                                   + sqrt(2) sqrt(%pi) x        %e

     To apply the Stirling formula only to terms that involve the
     variable 'k', use an optional third argument; for example
          (%i7) makegamma(pochhammer(a,k)/pochhammer(b,k));
          (%o7) (gamma(b)*gamma(k+a))/(gamma(a)*gamma(k+b))
          (%i8) stirling(%,1, lambda([s], not(freeof(k,s))));
          (%o8) (%e^(b-a)*gamma(b)*(k+a)^(k+a-1/2)*(k+b)^(-k-b+1/2))/gamma(a)
     The terms 'gamma(a)' and 'gamma(b)' are free of 'k', so the
     Stirling formula was not applied to these two terms.

     To use this function write first 'load("stirling")'.