Find degree of non-polynomial expression in Matlab

Hi,

say i have an expression as such:

syms x
y = x^(-5/2)

how to find the exponent of x since y is not a polynomial. is it possible?

thanks

Answer 1

[Star Strider]

Unfortunately not.

In theory,

xpnt = log(y)/log(x)

should work because it’s a fundamental log identity for conversion between log bases. The Symbolic Math Toolbox simply returns the symbolic expression, not the exponent.

Maybe in a future release...

Answer 2

[Christopher Creutzig]

That depends on what exactly you are looking for in the general case. Here's something that works in this particular one:

>> simplify(log(y)/log(x),'IgnoreAnalyticConstraints',true)

ans =

-5/2

(The result is valid in general, but simplification succeeds along a way that ignores branch cuts and is not universally valid.)

If your expression gets more complicated, you may need to use MuPAD functions directly:

>> feval(symengine,'ldegree',feval(symengine,'series',y,x))

ans =

-5/2

(If you want to look at the output of the feval(symengine,'series',y,x) part, I heartily recommend the pretty function.