But now, if one does interpret each of these terms in this way, the joint probability takes negative values roughly every λ λ d a : I 12 ( x ) = [ f 1 ∗ ∗ ( x ) f 2 ( x ) + f 1 ( x ) f 2 ∗ ∗ ( x ) ] = 1 2 d N d t d / π π d 2 + ( x − − a / 2 ) 2 d 2 + ( x + a / 2 ) 2 2 cos ⁡ ⁡ [ ( h / λ λ ) ( d 2 + ( x + a / 2 ) 2 − − d 2 + ( x − − a / 2 ) 2 ) ]