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Enhanced limit and series facilities

Asymptotic series have been extended to allow exponential terms to appear in the resulting series. Example:


    > asympt( Psi(2*exp(x))-x, x, 4 );

                               1           1            1
                   ln(2) - -------- - ---------- + O(-------)
                           4 exp(x)            2           4
                                      48 exp(x)      exp(x)


    > limit( Psi(2*exp(x))-x, x=infinity );

                                     ln(2)

Series expansions for erfc(x), GAMMA(a,x) for and MeijerG(a,b,x) for , have been added. Also asymptotic expansions for GAMMA(x) and binomial(n,k). Examples:


    > asympt( GAMMA(x)*Ei(x)/x^x/sqrt(2*Pi), x );

         1       13        601       319721        60973877         1
       ---- + ------- + -------- + ---------- + ------------- + O(-----)
        3/2       5/2        7/2          9/2            11/2      13/2
       x      12 x      288 x      51840 x      2488320 x         x


    # An indefinite summation using Gosper's algorithm
    > s := sum( binomial(2*n,n)/(n+1)/(2^n)^2, n );

                                     binomial(2 n, n)
                          s := - 1/2 ----------------
                                          (n - 1) 2
                                        (2       )


    > asympt(s,n,4);

                    2             1              1             1
             - ---------- + ------------ - ------------- + O(----)
                 1/2  1/2       1/2  3/2        1/2  5/2      7/2
               Pi    n      4 Pi    n      64 Pi    n        n


    # Hence since s = -2 at n = 0, we have
    > sum( binomial(2*n,n)/(n+1)/(2^n)^2, n=0..infinity );

                                       2


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