question: @ Override public double op ( double z ) { z = ( z - average ) / sigma ; double result = 0.5 * ( 1.0 + errorFunction . op ( z * Constants . M_SQRT_2 ) ) ; if ( result <= 1e-8 ) { double sum = 1.0 ; final double zsqr = z * z ; double i = 1.0 , g = 1.0 , x , y , a = Constants . QL_MAX_REAL , lasta ; do { lasta = a ; x = ( 4.0 * i - 3.0 ) / zsqr ; y = x * ( ( 4.0 * i - 1 ) / zsqr ) ; a = g * ( x - y ) ; sum -= a ; g *= y ; ++ i ; a = Math . abs ( a ) ; } while ( lasta > a && a >= Math . abs ( sum * Constants . QL_EPSILON ) ) ; result = - gaussian . op ( z ) / z * sum ; } return result ; }
target:  Computes the cumulative normal distribution. <p> Asymptotic expansion for very negative z as references on M. Abramowitz book.