“…Due to the question of Ulam and the answer of Hyers the stability of equations is called after their names. Later, a large number of papers and books have been published in connection with various generalizations of Hyers-Ulam theorem [5,7,10,13,15,17]. For instance, S.-E. Takahasi, H. Takagi, T. Miura and S. Miyajima [15] investigated the Hyers-Ulam stability constant K T h of linear differential operator (T h u)(t) = u (t) + h(t)u(t) and pointed out that it would be interesting to investigate the properties of the mapping h → K T h .…”