Forwarded message: > > > > > System is working correctly. A negative number cannot be raised to a > > > fractional power: > > > > > > > (-2)^(1/5) > > > [1] NaN
Well, maybe we should say that _R_ can't raise a negative number to a fractional power. Neither I nor the TI calculators have any trouble doing it;-) I'd say R is funtioning as designed, but it was designed to respond INcorrectly;-) This R exchange > (-2)^5 [1] -32 shows that -2 is a fifth root of -32 while this exchange > (-32)^(1/5) [1] NaN shows that R cannot find that root. The various suggestions for dealing with this amount to asking R a different question which we know has the same answer as the intended question (which R won't answer). On one level you could view this as a coding/implementation issue. I have not looked at R's code, but the usual computer way to handle exponents involves taking the log of the argument. This does not return the correct answer when the argument is negative. That's annoying. The TI graphing calculators were developed with an incredible amount of input from secondary math. teachers. They complained loudly about calculators returning wrong answers or non-answers to problems to which students knew the right answers. TI did a LOT of work on this. I wish R (and lots of scientific software) would do likewise. On another level, involving exponentiation is not entirely avoidable. For rational numbers like 1/5 we can (and usually do) interpret (-32)^(1/5) as a name for a real number that when raised to the fifth power gives -32. Another name for one such number is -2. But if we want to use an irrational exponent, say (-32)^pi we can't interpret it that way. (How do we multiply pi numbers together?) So eventually we have to either exponentiate or have a funciton that is undefined at many points. At lesat in theory. As far as computers and calculators are concerned, they cannot represent irrational numbers anyway -- eveything is a rational approximation. So I think the defect is in R, not in the original posted question. -------> First-time AP Stats. teacher? Help is on the way! See http://courses.ncssm.edu/math/Stat_Inst/Stats2007/Bob%20Hayden/Relief.html _ | | Robert W. Hayden | | 614 Nashua Street #119 / | Milford, New Hampshire 03055 USA | | | | email: bob@ the site below / x | website: http://statland.org | / '''''' _______________________________________________ R-sig-teaching@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-sig-teaching