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a(1)= because 3*10^1-1 = 29 and 3*10^1+1 = 31 are primes.
a(2)= because 6*10^1-1 = 59 and 6*10^1+1 = 61 are primes.
a(3)= because 6*10^2-1 = 599 and 6*10^2+1 = 601 are primes.
a(4)= because 3*10^3-1 = 2999 and 3*10^3+1 = 3001 are primes.
a(5)= because 9*10^3-1 = 8999 and 9*10^3+1 = 9001 are primes.
a(6)= because 3*10^7-1 = 29999999 and 3*10^7+1 = 30000001 are primes.
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