NAME:
	pdm
 PURPOSE: (one line)
	Period search by phase dispersion minimization ($\theta$ based).
 DESCRIPTION:
	This routine computes the Theta statistic for period searching
	in time-series data using the technique described by Stellingwerf,
	ApJ, 224, pp. 953-960 (1978).
 CATEGORY:
  Photometry
 CALLING SEQUENCE:
	pdm,t,x,sigx0,freq1,freq2,dfreq,freq,period,theta
 INPUTS:
	t     - independent variable (usually time)
	x0    - dependent variable (usually magnitude or intensity)
	sigx0 - Uncertainty on x.
	freq1 - Lower limit to frequency to compute statistic  (units=[1/t])
	freq2 - Upper limit to frequency to compute statistic  (units=[1/t])
	dfreq - Frequency interval.  (units=[1/t])
 OPTIONAL INPUT PARAMETERS:
 KEYWORD PARAMETERS:
 OUTPUTS:
	freq  - Frequency vector.
	period - inverse frequency.
	theta - PDM statistic (1 ==> insignificant, 0 ==> significant).
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
	Written 1992 Feb 11, by Marc W. Buie, Lowell Observatory