“…the skew-symmetric matrix f (B)−f (B) tr can have nonzero entries only in (1, 2), (1, 3), (2, 1), (3,1) positions, and the absolute value of these entries cannot exceed µ. On the other hand, W (f (B)) = W (B), which is known to be the circular disk centered at zero with radius µ/ √ 2 (see [16] or [11,Theorem 4.1], for example), and therefore ±iµ √ 2 are eigenvalues of the matrix f (B) − f (B)…”