R. Balasubramanian and V. Kumar-murty, Zeros of Dirichlet L-functions, Ann. Scient. Ecole Norm. Sup, vol.25, pp.567-615, 1992.

M. B. Barban and P. P. Vehov, On an extremal problem, Trans. Moscow Math. Soc, vol.18, pp.91-99, 1968.

S. Graham, An asymptotic estimate related to Selberg's sieve, J. Number Theory, vol.10, pp.83-94, 1978.

S. Graham, On Linnik's constant, Acta Arith, vol.39, pp.163-179, 1981.

M. Jutila, On Linnik's constant, Math. Scand, vol.41, pp.45-62, 1977.

Y. Motohashi, On a problem in the theory of sieve methods, Kokyuroku RIMS, vol.222, pp.9-50, 1974.

V. Kumar, , vol.40