function prd2octv(args)
*: prd2octv period k0 で周期を，waveletのオクターブに変換する．結果はresultに入る．見延
*usage: octv2prd period k0 
* With given period,
* This Grads Script returns an octave value corresponding to the given period,
* in the case of k0=6. Notice if k0 is not equal to 6, this script returns
* INAPPROPRIATE value.
* function main(args)
* ascale=2.^oscale;
* period = ascale;
* period = 2^oscale *(4*pi)/(k0 + sqrt(2 + k0^2));
* ->
* 2^oscale=preiod/{(4*pi)/(k0 + sqrt(2 + k0^2))}=rh
* oscale=log2(rh)=log(rh)/log(2)
*
period=subwrd(args,1)
k0=subwrd(args,2)
'run sqrt '%2+k0*k0
rh=period/(4*3.1416/(k0+result))
'run logn '%rh; num=result
'run logn 2';   den=result
octave=num/den
return octave