[Gretl-users] Re: Negative numbers to fractional powers

On Wed, 12 Aug 2020, Alecos Papadopoulos wrote: > Now, consider series "Z" which has both positive /and/ negative > values. We want to compute > > W = Z^(1/3) > > Given a recent new feature in gretl, the sign function sgn(), I guess > we can avoid the obvious conditional if-else statement, and compute > W by writing > > W = sgn(Z) * (abs(Z)^(1/3)) > > that covers also the case of Z taking an exact zero value. > > Is this the proper/efficient way? My second reply. No, this is not proper! <gretl> ? Z = -2 Generated scalar Z = -2 ? W = sgn(Z) * (abs(Z)^(1.0/4)) Generated scalar W = -1.18921 ? eval W^4 2 </gretl> <octave> octave:1> Z = -2 Z = -2 octave:2> W = Z^(1.0/4) W =  0.84090 + 0.84090i octave:3> W^4 ans = -2.0000e+00 + 1.2561e-15i <octave> There's a digital approximation involved in the octave answer, via complex numbers, but it's basically right, while the answer according to your proposal is flat wrong. Allin _______________________________________________ Gretl-users mailing list -- gretl-users(a)gretlml.univpm.it To unsubscribe send an email to gretl-users-leave(a)gretlml.univpm.it Website: https://gretlml.univpm.it/postorius/lists/gretl-users.gretlml.univpm.it/