“…An even easier way of seeing this is by noting that −∆ A(| · |) = 2δ(·) (or 2δ(·) − δ(· − 1) − δ(· + 1) if A(| · |) is extended by zero outside of Ω − Ω), in the sense of distributions. This can then be used in(7) immediately resulting in (u, v) A = (u, −∆ A(| · |) * v) L 2 (R) = 2(u, v) L 2 (R) = 2(u, v) L 2 (Ω) (or, respectively, in (u, v) A = (u, −∆ A(| · |) * v) L 2 (R) = 2(u, v) L 2 (R) − (u, v(· − 1)) L 2 (R) − (u, v(· + 1)) L 2 (R) = 2(u, v) L 2 (Ω) ).…”