An information source generates a binary sequence {α_n}. α_n can take one of the two possible values -1 and +1 with equal probability and are statistically independent and identically distributed. This sequence is precoded to obtain another sequence {β_n}, as β_n=α_n+kα_n-3. The sequence {β_n} is used to modulate a pulse g(t) to generate the baseband signal X(t)=∑_n=-∞^∞β_ng(t-nT), where g(t)=1, 0≤t≤T and 0, otherwise. If there is a null at f=1/(3T) in the power spectral density of X(t), then k is