The convert(expr, Sum) command attempts representing the given  as a "formal power series", expressed using the inert Sum command. For that purpose, two approaches are used: one is a Global approach, which aims at representing the whole expression as a Sum of terms of the form , with a representing a constant and c[n] a coefficient not depending on x, the main variable; another one is Local and consists of replacing, in , occurrences of mathematical functions in by their Sum representations - in this case the terms summed are of the generic form , where f(x) is a mapping of x not involving functions for which a sum representation is known. The conversion to Sum does not include computing the sums.

All the optional arguments accepted by the Maple convert/to_special_function network are allowed, so that one can restrict the application of convert to Sum in different manners.

The convert(expr, sum) command does the same as convert(expr, Sum) followed by an attempt to compute all the Sums introduced, and here again to override the default behavior you can use any of the optional arguments described in convert/to_special_function.

By default, a combination of the Global and Local methods is used; in this approach, when possible, all mathematical functions found expr are converted, and in cases of nested mathematical functions, or linear combination of functions which admit a hypergeometric representation, a Global conversion is attempted first, taking advantage of the properties of such expressions. By invoking this conversion with the optional argument method = Global (or skip = Local), or with  method = Local (or skip = Global), respectively only the Global or the Local methods are used.

NOTE: The use of the following optional arguments automatically make convert/Sum to skip the Local method and only work using the Global method: x = a, makereal = true, differentialorder = n, dir = value, method = value, indicating one of the methods rational, exponential, hypergeometric, and recurrence = value. Also, powers of x are not converted unless you explicitly request that using the optional argument include = powers (all powers) or include = radicals (only fractional powers).

The implementation for Global conversions is based on references [1] and [2] (see at the end), and mainly expands meromorphic expressions of certain type (see next paragraph) into their corresponding Laurent-Puiseux series as a Sum of terms of the form , where  is called the symmetry number,  is the shift number, and  is the expansion point. If the expansion point is not given, then an expansion around the origin is computed; if the expansion point is at infinity, then the command searches for an asymptotic (possibly divergent) series. This implementation also works for formal Laurent-Puiseux series, and in certain cases of logarithmic singularities.

The algorithm: given expr to be represented as a Sum, consists of first representing  as a homogeneous linear differential equation with polynomial coefficients with appropriate initial conditions, an ODE-IVP (see gfun[holexprtodiffeq], PDEtools[dpolyform]), then finding a formal power series for it (see Slode,FPseries, dsolve, formal_series, dsolve, formal_solution).

The types of expressions that Global conversions can handle are: - expressions of hypergeometric type, where b(k+m)/b(k) is a rational function of k for some integer m; - expressions of exponential type, which satisfy a linear homogeneous differential equation with constant coefficients; - expressions of rational type, which are either rational or have a rational derivative; - linear combinations of hypergeometric functions are treated by the Petkovsek-van-Hoeij algorithm; see LREtools[hypergeomsols].

differentialorder: a positive integer n (default: n=4); upper bound for the order of the differential equation searched for. This controls the depth of the search for a differential equation for expr. Higher values of n will increase the chance to find the solution, but increase the running time as well.

dir: one of default, left, right, real, or complex; direction of the limit computation for initial values. If a, the expansion point, is finite, then the default is dir = complex. If a is either infinity or -infinity, then the default is dir = real. See also limit. This optional argument can be used to request real or one-sided (e.g. asymptotic) series.

makereal: either true or false (default); makereal = true (or just makereal for short) indicates that a series with real coefficients should be returned.

method: one of default, hypergeometric, rational, or exponential. Specifies the method that will be used; the default method uses an internal selection strategy. See the Examples below for an illustration of the various methods.

recurrence: either true or false (default). If recurrence = true (or recurrence for short) is given and the coefficients entering the formal power series can be computed in closed form, then the output is a recurrence for b(k).

The 7 functions in the "Ei_related" class are:

* Partial match of "sum" against topic "sum_form".

`diff/g` := proc(a,x) g(a)*diff(a,x) end proc;

* Partial match of "sum" against topic "sum_form".

* Partial match of "sum" against topic "sum_form".

* Partial match of "sum" against topic "sum_form".

[1] Wolfram Koepf, Power series in computer algebra. Journal of Symbolic Computation 13, 1992, 581-603

[2] Dominik Gruntz, Wolfram Koepf: Maple package on formal power series. Maple Technical Newsletter 2 (2), 1995, 22-28