Need pow(-1,1.2) to be 1

Question

I am using math.h with GCC and GSL. I was wondering how to get this to evaluate?

I was hoping that the pow function would recognize pow(-1,1.2) as ((-1)^6)^(1/5). But it doesn't.

Does anybody know of a c++ library that will recognize these? Perhaps somebody has a decomposition routine they could share.

Answer 1

It seems like you're looking for pow(abs(x), y).

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Explanation: you seem to be thinking in terms of

x^y = (x^N)^(y/N)

If we choose that N === 2, then you have

(x²)^y/2 = ((x²)^1/2)^y

But

(x²)^1/2 = |x|

Substituting gives

|x|^y

This is a stretch, because the above manipulations only work for non-negative x, but you're the one who chose to use that assumption.