wt_std_error_AR1MA1 = function(n,c0,r1,r2){
#
# Returns an approximate standard error for a ARIMA(1,d,1) model
#
# Written by:
# -- 
# John L. Weatherwax                2009-04-21
# 
# email: wax@alum.mit.edu
# 
# Please send comments and especially bug reports to the
# above email address.
# 
#-----

sqrt( (c0/n) * ( 1 + ( 2*r1^2 )/( r1 - r2 ) ) ) 

}