Perfect powers k (exponent greater than 1) such that k-1 and k+1 are both semiprime.

Terms

a(0) =144a(1) =216a(2) =900a(3) =1764a(4) =2048a(5) =3600a(6) =10404a(7) =11664a(8) =39204a(9) =97344a(10) =213444a(11) =248832a(12) =272484a(13) =360000a(14) =656100a(15) =685584a(16) =1040400a(17) =1102500a(18) =1127844a(19) =1633284a(20) =2108304a(21) =2214144a(22) =3504384a(23) =3802500a(24) =4112784a(25) =4536900a(26) =4588164a(27) =5475600a(28) =7784100a(29) =7851204