Functions and Variables for stirling
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 -- Function: stirling (Z,N)
 -- Function: 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").

     Gamma and factorial functions Share packages Package stirling