mtaylor MuPAD-Matlab [英] mtaylor MuPAD - Matlab

问题描述

我正在尝试从MatLab的MuPAD引擎运行函数mtaylor，该引擎提供函数的多变量泰勒展开.但是，它总是告诉我我正在尝试扩展无效点.这是我尝试过的最小工作示例:

I am trying to run the function mtaylor from the MuPAD engine in MatLab, which provides a multivariate Taylor expansion of a function. However, it keeps telling me I am trying to expand around an invalid point. Here is a minimal working example of what I tried:

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), [x, y], 4)

Error message:
 vError using mupadengine/feval (line 157)
 MuPAD error: Error: Invalid expansion point. [mtaylor]

为什么这行不通?

推荐答案

与MuPAD的 mtaylor

The reason this works with MuPAD's mtaylor

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), x, 4)  % [x] is fine too

这不是

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), [x, y], 4)

是[x, y]自变量被视为单个符号向量自变量/变量，而不是用于扩展的变量列表.您的表达式exp(x^2 - y)并不是矢量变量，而是简单的标量x和y.

is that the [x, y] argument is seen as a single symbolic vector argument/variable rather than a list of variables for the expansion. Your expression, exp(x^2 - y), is not in terms of vector variables, but simple scalars, x and y.

解决方法是将列表作为字符串传递:

The workaround is to pass the list as a string:

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), '[x, y]', 4)

或

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), ['[' char(x) ',' char(y) ']'], 4)

或使用 evalin 将MuPAD命令写为单个字符串，如下所示: @Daniel在评论中建议:

or use evalin to write the MuPAD command a single string as suggested by @Daniel in the comments:

syms x y;
evalin(symengine,'mtaylor(exp(x^2 - y), [x, y], 4)')

MuPAD中的数组和矩阵与列表
为了进一步说明，Matlab中的符号变量数组对应于MuPAD array 类型，可以通过feval(symengine,'array','1..1','1..2','[x,y]')创建.更具体地说，它们的类型为 Dom::Matrix() ，通过V=feval(symengine,'Dom::Matrix()','[x,y]')或仅通过syms x y; V=[x,y]创建.

Arrays and Matrices vs. Lists in MuPAD
For further clarification, arrays of symbolic variables in Matlab correspond to the MuPAD array type, which can be created via feval(symengine,'array','1..1','1..2','[x,y]'). More specifically, they are of type Dom::Matrix(), which can be created via V=feval(symengine,'Dom::Matrix()','[x,y]') or just syms x y; V=[x,y].

mtaylor函数需要一个列表输入，可以通过L=evalin(symengine,'[x,y]')创建.因此

The mtaylor function requires a list input, which can be created via L=evalin(symengine,'[x,y]'). Thus

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), V, 4)

将产生与您的问题相同的错误，但是

will produce the same error as in your question, but

syms x y;
feval(symengine,'mtaylor',exp(x^2 - y), L, 4)

将正常工作.不幸的是，L和V在Matlab中看起来是相同的，但是您可以使用MuPAD的 domtype 函数来区分它们:

will work correctly. Unfortunately, L and V appear identical from within Matlab, but you can use MuPAD's domtype function to differentiate them:

feval(symengine,'domtype',V)
feval(symengine,'domtype',L)

返回Dom::Matrix()和DOM_LIST.

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