question: public double [ ] [ ] extractMainSetJacobian ( final ODEState state ) { final double [ ] p = state . getSecondaryState ( index ) ; final double [ ] [ ] dYdY0 = new double [ jode . getDimension ( ) ] [ jode . getDimension ( ) ] ; int j = 0 ; for ( int i = 0 ; i < jode . getDimension ( ) ; i ++ ) { System . arraycopy ( p , j , dYdY0 [ i ] , 0 , jode . getDimension ( ) ) ; j += jode . getDimension ( ) ; } return dYdY0 ; }
target:  Extract the Jacobian matrix with respect to state.