Octagonal numbers (A000567) that are semiprimes (A001358).

Terms

a(0) =21a(1) =65a(2) =133a(3) =341a(4) =481a(5) =1541a(6) =4033a(7) =5461a(8) =6533a(9) =8321a(10) =11041a(11) =13333a(12) =14981a(13) =31621a(14) =38081a(15) =48133a(16) =56033a(17) =79381a(18) =83333a(19) =97921a(20) =109061a(21) =111361a(22) =133141a(23) =188501a(24) =197633a(25) =206981a(26) =219781a(27) =229633a(28) =256961a(29) =282133