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# Series

## Introduction to Series

Maxima contains functions `Taylor` and `Powerseries` for finding the series of differentiable functions. It also has tools such as `Nusum` capable of finding the closed form of some series. Operations such as addition and multiplication work as usual on series. This section presents the various global various variables which control the expansion.

## Definitions for Series

Variable: CAUCHYSUM
  default: [FALSE] - When multiplying together sums with INF as their upper limit, if SUMEXPAND is TRUE and CAUCHYSUM is set to TRUE then the Cauchy product will be used rather than the usual product. In the Cauchy product the index of the inner summation is a function of the index of the outer one rather than varying independently. That is: SUM(F(I),I,0,INF)*SUM(G(J),J,0,INF) becomes SUM(SUM(F(I)*G(J-I),I,0,J),J,0,INF)

Function: DEFTAYLOR (function, exp)
  allows the user to define the Taylor series (about 0) of an arbitrary function of one variable as exp which may be a polynomial in that variable or which may be given implicitly as a power series using the SUM function. In order to display the information given to DEFTAYLOR one can use POWERSERIES(F(X),X,0). (see below).

      (%i1) DEFTAYLOR(F(X),X**2+SUM(X**I/(2**I*I!**2),
              I,4,INF));
      (%o1)                          [F]
      (%i2) TAYLOR(%E**SQRT(F(X)),X,0,4);
                           2         3          4
                          X    3073 X    12817 X
      (%o2)/R/     1 + X + -- + ------- + -------- +  . . .
                          2     18432     307200

Variable: MAXTAYORDER
  default: [TRUE] - if TRUE, then during algebraic manipulation of (truncated) Taylor series, TAYLOR will try to retain as many terms as are certain to be correct.

Function: NICEINDICES (expr)
  will take the expression and change all the indices of sums and products to something easily understandable. It makes each index it can "I" , unless "I" is in the internal expression, in which case it sequentially tries J,K,L,M,N,I0,I1,I2,I3,I4,... until it finds a legal index.

Variable: NICEINDICESPREF
  default: [I,J,K,L,M,N] - the list which NICEINDICES uses to find indices for sums and products. This allows the user to set the order of preference of how NICEINDICES finds the "nice indices". E.g. NICEINDICESPREF:[Q,R,S,T,INDEX]$. Then if NICEINDICES finds that it cannot use any of these as indices in a particular summation, it uses the first as a base to try and tack on numbers. Here, if the list is exhausted, Q0, then Q1, etc, will be tried.

Function: NUSUM (exp,var,low,high)
  performs indefinite summation of exp with respect to var using a decision procedure due to R.W. Gosper. exp and the potential answer must be expressible as products of nth powers, factorials, binomials, and rational functions. The terms "definite" and "indefinite summation" are used analogously to "definite" and "indefinite integration". To sum indefinitely means to give a closed form for the sum over intervals of variable length, not just e.g. 0 to inf. Thus, since there is no formula for the general partial sum of the binomial series, NUSUM can't do it.

Function: PADE (taylor-series,num-deg-bound,denom-deg-bound)
  returns a list of all rational functions which have the given taylor-series expansion where the sum of the degrees of the numerator and the denominator is less than or equal to the truncation level of the power series, i.e. are "best" approximants, and which additionally satisfy the specified degree bounds. Its first argument must be a univariate taylor-series; the second and third are positive integers specifying degree bounds on the numerator and denominator. PADE's first argument can also be a Laurent series, and the degree bounds can be INF which causes all rational functions whose total degree is less than or equal to the length of the power series to be returned. Total degree is num-degree + denom-degree. Length of a power series is "truncation level" + 1 - minimum(0,"order of series").

      (%i15) ff:taylor(1+x+x^2+x^3,x,0,3);
                                           2    3
      (%o15)/T/                    1 + X + X  + X  + . . .
      (%i16) pade(ff,1,1);
                                              1
      (%o16)                              [- -----]
                                            X - 1
      (%i1) ff:taylor(-(83787*X^10-45552*X^9-187296*X^8
                        +387072*X^7+86016*X^6-1507328*X^5
                        +1966080*X^4+4194304*X^3-25165824*X^2
                        +67108864*X-134217728)
             /134217728,x,0,10);
      (%i25) PADE(ff,4,4);
      (%o25) []

  There is no rational function of degree 4 numerator/denominator, with this power series expansion. You must in general have degree of the numerator and degree of the denominator adding up to at least the degree of the power series, in order to have enough unknown coefficients to solve.

      (%i26) PADE(ff,5,5);
      (%o26) [-(520256329*X^5-96719020632*X^4-489651410240*X^3
                 -1619100813312*X^2 -2176885157888*X-2386516803584)
      	/(47041365435*X^5+381702613848*X^4+1360678489152*X^3
                      +2856700692480*X^2
      	        +3370143559680*X+2386516803584)]

Variable: POWERDISP
  default: [FALSE] - if TRUE will cause sums to be displayed with their terms in the reverse order. Thus polynomials would display as truncated power series, i.e., with the lowest power first.

Function: POWERSERIES (exp, var, pt)
  generates the general form of the power series expansion for exp in the variable var about the point pt (which may be INF for infinity). If POWERSERIES is unable to expand exp, the TAYLOR function may give the first several terms of the series. VERBOSE[FALSE] - if TRUE will cause comments about the progress of POWERSERIES to be printed as the execution of it proceeds.

      (%i1) VERBOSE:TRUE$
      (%i2) POWERSERIES(LOG(SIN(X)/X),X,0);
      Can't expand
                                       LOG(SIN(X))
      So we'll try again after applying the rule:
                                              d
                                            / -- (SIN(X))
                                            [ dX
                              LOG(SIN(X)) = I ----------- dX
                                            ]   SIN(X)
                                            /
      In the first simplification we have returned:
                                   /
                                   [
                                   I COT(X) dX - LOG(X)
                                   ]
                                   /
                          INF
                          ====        I1  2 I1             2 I1
                          \      (- 1)   2     BERN(2 I1) X
                           >     ------------------------------
                          /                I1 (2 I1)!
                          ====
                          I1 = 1
      (%o2)                -------------------------------------
                                            2

Variable: PSEXPAND
  default: [FALSE] - if TRUE will cause extended rational function expressions to display fully expanded. (RATEXPAND will also cause this.) If FALSE, multivariate expressions will be displayed just as in the rational function package. If PSEXPAND:MULTI, then terms with the same total degree in the variables are grouped together.

Function: REVERT (expression,variable)
  Does reversion of Taylor Series. "Variable" is the variable the original Taylor expansion is in. Do LOAD(REVERT) to access this function. Try

      REVERT2(expression,variable,hipower)

  also. REVERT only works on expansions around 0.

Function: SRRAT (exp)
  this command has been renamed to TAYTORAT.

Function: TAYLOR (exp, var, pt, pow)
  expands the expression exp in a truncated Taylor series (or Laurent series, if required) in the variable var around the point pt. The terms through (var-pt)**pow are generated. If exp is of the form f(var)/g(var) and g(var) has no terms up to degree pow then TAYLOR will try to expand g(var) up to degree 2*pow. If there are still no non-zero terms TAYLOR will keep doubling the degree of the expansion of g(var) until reaching pow*2**n where n is the value of the variable TAYLORDEPTH[3]. If MAXTAYORDER[FALSE] is set to TRUE, then during algebraic manipulation of (truncated) Taylor series, TAYLOR will try to retain as many terms as are certain to be correct. Do EXAMPLE(TAYLOR); for examples. TAYLOR(exp,[var1,pt1,ord1],[var2,pt2,ord2],...) returns a truncated power series in the variables vari about the points pti, truncated at ordi. PSEXPAND[FALSE] if TRUE will cause extended rational function expressions to display fully expanded. (RATEXPAND will also cause this.) If FALSE, multivariate expressions will be displayed just as in the rational function package. If PSEXPAND:MULTI, then terms with the same total degree in the variables are grouped together. TAYLOR(exp, [var1, var2, . . .], pt, ord) where each of pt and ord may be replaced by a list which will correspond to the list of variables. that is, the nth items on each of the lists will be associated together. TAYLOR(exp, [x,pt,ord,ASYMP]) will give an expansion of exp in negative powers of (x-pt). The highest order term will be (x-pt)^(-ord). The ASYMP is a syntactic device and not to be assigned to. See also the TAYLOR_LOGEXPAND switch for controlling expansion.

Variable: TAYLORDEPTH
  default: [3] - If there are still no non-zero terms TAYLOR will keep doubling the degree of the expansion of g(var) until reaching pow*2**n where n is the value of the variable TAYLORDEPTH[3].

Function: TAYLORINFO (exp)
  returns FALSE if exp is not a Taylor series. Otherwise, a list of lists is returned describing the particulars of the Taylor expansion. For example,

      (%i3) TAYLOR((1-Y^2)/(1-X),X,0,3,[Y,A,INF]);
                   2                        2
      (%o3)/R/ 1 - A  - 2 A (Y - A) - (Y - A)
                          2                        2
                  + (1 - A  - 2 A (Y - A) - (Y - A) ) X
               2                        2   2
       + (1 - A  - 2 A (Y - A) - (Y - A) ) X
                          2                        2   3
                  + (1 - A  - 2 A (Y - A) - (Y - A) ) X
           + . . .
      (%i4) TAYLORINFO(%o3);
      (%o4)                       [[Y, A, INF], [X, 0, 3]]

Function: TAYLORP (exp)
  a predicate function which returns TRUE if and only if the expression 'exp' is in Taylor series representation.

Variable: TAYLOR_LOGEXPAND
  default: [TRUE] controls expansions of logarithms in TAYLOR series. When TRUE all log's are expanded fully so that zero-recognition problems involving logarithmic identities do not disturb the expansion process. However, this scheme is not always mathematically correct since it ignores branch information. If TAYLOR_LOGEXPAND is set to FALSE, then the only expansion of log's that will occur is that necessary to obtain a formal power series.

Variable: TAYLOR_ORDER_COEFFICIENTS
  default: [TRUE] controls the ordering of coefficients in the expression. The default (TRUE) is that coefficients of taylor series will be ordered canonically.

Function: TAYLOR_SIMPLIFIER
  - A function of one argument which TAYLOR uses to simplify coefficients of power series.

Variable: TAYLOR_TRUNCATE_POLYNOMIALS
  default: [TRUE] When FALSE polynomials input to TAYLOR are considered to have infinite precison; otherwise (the default) they are truncated based upon the input truncation levels.

Function: TAYTORAT (exp)
  converts exp from TAYLOR form to CRE form, i.e. it is like RAT(RATDISREP(exp)) although much faster.

Function: TRUNC (exp)
  causes exp which is in general representation to be displayed as if its sums were truncated Taylor series. E.g. compare EXP1:X^2+X+1; with EXP2:TRUNC(X^2+X+1); . Note that IS(EXP1=EXP2); gives TRUE.

Function: UNSUM (fun,n)
  is the first backward difference fun(n) - fun(n-1).

      (%i1) G(P):=P*4^N/BINOMIAL(2*N,N);
                                                  N
                                               P 4
      (%o1)                       G(P) := ----------------
                                         BINOMIAL(2 N, N)
      (%i2) G(N^4);
                                           4  N
                                          N  4
      (%o2)                           ----------------
                                     BINOMIAL(2 N, N)
      (%i3) NUSUM(%o2,N,0,N);
                               4        3       2              N
                2 (N + 1) (63 N  + 112 N  + 18 N  - 22 N + 3) 4      2
      (%o3)      ------------------------------------------------ - ------
                              693 BINOMIAL(2 N, N)                 3 11 7
      (%i4) UNSUM(%,N);
                                           4  N
                                          N  4
      (%o4)                           ----------------
                                     BINOMIAL(2 N, N)

Variable: VERBOSE
  default: [FALSE] - if TRUE will cause comments about the progress of POWERSERIES to be printed as the execution of it proceeds.

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