Indexing in symsum-Funktion

Hello,

how do I index the A in a function like this?

anz = 5;
A = sym('A',[anz 1]);
q = symsum( A(vpa(n))*r^n  * cos(n*phi) ,'n' ,2,anz)

The error message in this case is:

??? Error using ==> mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression into a double array.

If the input expression contains a symbolic variable, use the VPA function instead.

Can anyone help me? Thanks in advance. Daniel

Answer 1

[Walter Roberson]

I think you are missing

syms n

before this code.

I do not see any reason to use vpa(n) there:

syms n;
q = symsum( A(n)*r^n * cos(n*phi), n, 2, anz );

[Daniel Thoden]

Thanks, but unfortunately it didn't work out! Same error message occurs!

[Walter Roberson]

Try,

temp = subs('add(A[n]*r^n * cos(n*phi), n=2..anz)', {'A', 'r', 'phi'}, {A, r, phi});

q = evalin(symengine, temp);

Notice that I have used "add" instead of "symsum", and have used [] for indexing instead of (), and that I have used "n=2..anz" instead of "n, 2, anz" : these changes rephrase the question into something executable in MuPad directly, and the evalin() then executes in MuPad.

symsum() corresponds to the MuPad sum() call, which is intended for symbolic summation (that is, where one or both of the endpoints is a symbolic variable); when the bounds of the summation are known integers, then the MuPad add() command is the appropriate one. There are a number of boundary cases where sum() and add() produce different answers: sum() works by trying to find the closed-form formula for the summation and then to evaluate it at the upper bound and subtract off the closed-form evaluated at the lower bound, but that doesn't work if there are singularities or if the closed form evaluates to infinity (infinity minus infinity being undefined.)

[Christopher Creutzig]

MuPAD does not have an “add” function, that's Maple syntax. Just use sum - I don't think creating cases with such differences is easy in MuPAD (which tends to use direct summation in many of the cases you mentioned), but if you do, the proper syntax would be _plus(f(k) $ k=a..b). Which, incidentally, is also a bit faster than sum.

[Walter Roberson]

Thanks, Christopher. The MuPad documentation available online tends not to talk about these kind of nuances.

I do not have the Symbolic Toolkit so it is not uncommon for me to slip in to Maple practices.

[Aaron J. Hendrickson]

Walter. When I run:

syms n
anz = 5;
A = sym('A',[anz 1]);
q = symsum(A(n)*r^n*cos(n*phi), n, 2, anz);

I get an error in MATLAB saying:

Error using sym/subsindex (line 766)
Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and the body of the function is a SYM expression. When indexing, the input must be numeric, logical, or ':'.

Error in sym/subsref (line 805)
            R_tilde = builtin('subsref',L_tilde,Idx);

Not sure how to index variables in sums and products...do you know what is going on here?

Answer 2

[VBBV]

syms n r phi   % declare r and phi as symbolic also
anz = 5;
A = sym('A',[anz 1])

A = 

q = symsum(A.*r^n*cos(n*phi), n, 2, anz)  %  As error shows Matlab indexing is numeric & not symbolic 

q = 

[VBBV]

koAs A is symbolic vector , you can access the elements of that vector through integers and not using symbols e.g n as below

A(n)