  We have used the Date-Compensated Discrete Fourier Transform and Periodogram
analysis of the solar neutrino flux data from 1) 5-day-long samples from
Super-Kamiokande-I detector during the period from June, 1996 to July, 2001; 2)
10-day-long samples from the same detector during the same period and 3)
45-day-long samples from the same detector during the same period. (1) exhibits
periodicity around 0.21-0.22, 0.67-0.77, 1.15-1.98, 6.72-6.95, 12.05-13.24,
22.48-24.02, 33.50 and 40.73 months. (2) shows periodicity around 0.39-0.45,
1.31-2.23, 5.20-5.32, 9.43-11.65, 13.54-14.38, 24.54, 32.99 and 41.69 months.
For (3) we observe periodicity around 1.61, 14.01, 24.06, 32.50 and 42.03
months. We have found almost similar periods in the solar flares, sunspot data,
solar proton data (\bar{\epsilon}>10 Mev) which indicates that the solar
activity cycle may be due to the variable character of nuclear energy
generation inside the sun.
