\[x^{2} - 4x + 1 - a = 0\]
\[1 - a > 0\]
\[- a > - 1\]
\[a < 1.\]
\[Уравнение\ имеет\ два\ корня\ при\ D > 0:\]
\[D_{1} = 4 - (1 - a) = 4 - 1 + a = 3 + a;\]
\[3 + a > 0\]
\[a > - 3.\]
\[a = - 2;\ - 1;0.\]
\[Ответ:при\ \ a = 0.\]