[MATRIX-SIG] -3**(-1/3.), sgn()

[Jean-Bernard ADDOR]

Hy !

a is an array with positive, negatives or null values

I want to compute a**(-1/3.). It works perfectly if (a) is not an array
but a scalar (function pow). The funcion power do not give fractional
power from negative number. 

I would like to write sgn(a)*abs(a)**(-1/3.) but the sgn function seems
not to be defined in python (perhaps because it is not in c, but it is in
fortran). I can use greater_equal. Do you have a better solution ?

A bientot,

	Jean-Bernard



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