a(n) = 2^(2*prime(n) - 3) - 1.A139291a(n) = 2^(2*prime(n) - 3) - 1.Terms a(0) =1a(1) =7a(2) =127a(3) =2047a(4) =524287a(5) =8388607a(6) =2147483647a(7) =34359738367External referencesoeis: A139291