Abstract
We analyze the radio spectral index of blazars from the University of Michigan Radio Astronomy Observatory database, and find that there exists quasi-periodic activity in 2251+158. The long-term periodicity analysis is accomplished by three methods, which are the Jurkevich method (Jurkevich), the discrete correlation function (DCF), and the Periodogram method (P). The results show that 2251+158 has two strong periodicities, which are P1 = 6.3 ± 1.1 yr and P2 = 3.8 ± 1.2 yr.
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