Ian, On Mon, 2009-11-23 at 17:34 +0000, Ian Tickle wrote: > Ed, > > > For instance, if angles are measured in degrees and x<<1 > > sin x ~ pi * x / 180 > > sin x ~ x > > Your equations cannot simultaneously be true & in fact the 1st one is > obviously wrong, the 2nd is right. In the 1st case I think you meant > (substituting 'x*deg' for 'x' in your correct 2nd equation): >
Hmm... It's not the same x in these two equations - one is measured in degrees, the other in radians. > Just one: it's a<=b<=c. In any case, this comment is analogous to Henry > Ford's famous sales pitch for the Model T: "You can have a car in any > colour so long as it's black". Tell me, which would you say makes more > sense: a) 1 person spends 10 secs adding 10 lines to the syminfo file > once and for all, or b) many people post queries to CCP4BB about > re-indexing their MTZ files because the processing mis-identified 2-fold > screw axes from the systematic absences? > Tough call. On one hand, refusing P22121's right to exist is discrimination, on the other - these are the subtleties that help understanding so this has some educational value. Then there is Ockham's razor (which I personally believe people sometimes take too far). I think you pose the question in the way which pushes towards certain answer, let me try it differently: Which one makes more sense: 1) people learning more about space groups and reading the manuals of the software they are using to process data or 2) adding more space groups and using more paper to print the International Tables for Crystallography (gently hugs an imaginary tree)? Seriously though, I think it makes sense to keep just P21212, because you don't get a different crystal form by axes permutation. > > PS. By the way, did you notice that pi^2 ~ g ? I > > ... and did you notice that e^(i*pi) + 1 = 0 connects the 5 fundamental > mathematical constants? - that also has nothing whatsoever to do with > this thread ;-). > Oh yeah - e^(i*pi)=-1 is my favorite meditation object :-) Nicely connects arithmetics, geometry, calculus and complex analysis. Cheers, Ed. --
