question: public static double dchisqLog ( double x , double ncp ) { if ( ncp == 0.0 ) return dchisqLog ( x ) ; double xh = Math . sqrt ( x ) ; double mean = Math . sqrt ( ncp ) ; return Math . log ( dnorm ( xh - mean ) + dnorm ( - xh - mean ) ) - Math . log ( 2 * xh ) ; }
target:  Returns the log-density value of a noncentral Chi-square distribution.