_T_i_m_e-_S_e_r_i_e_s
	
	     ts(x, start, end, frequency)
	     as.ts(x)
	     is.ts(x)
	
	_A_r_g_u_m_e_n_t_s:
	
	           x : the observed time-series values.
	
	       start : the time of the first observation.
	
	         end : the time of the last observation.
	
	   frequency : the number of observations per unit of time.
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     ts creates a time series variable.  Time series
	     represent data which has been sampled at equispaced
	     points in time.
	
	     frequency is used when the series is sampled an integer
	     number of times in the unit time interval.  For exam-
	     ple, one could use a value of 7 for frequency when the
	     data are sampled daily, and the natural time period is
	     a week, or 12 when the data are sampled monthly and the
	     natural time period is a year.
	
	     start and end can either be integers which correspond
	     to natural time units, or vectors of two integers,
	     which give a natural time unit and a (1-based) number
	     of samples into the time unit.
	
	     as.ts and is.ts respectively coerce a vecor into a
	     time-series and test whether an object is a time
	     series.
	
	_S_e_e _A_l_s_o:
	
	     time, start, end, frequency.
	
	_E_x_a_m_p_l_e_s:
	
	     # start date - July 1954
	     x <- ts(x, start=c(1954,7), frequency=12)