Composite numbers requiring increasingly larger bases to become prime by base reversal.

Terms

a(0) =4a(1) =6a(2) =8a(3) =9a(4) =16a(5) =27a(6) =36a(7) =78a(8) =81a(9) =102a(10) =114a(11) =144a(12) =270a(13) =420a(14) =480a(15) =750a(16) =1848a(17) =2382a(18) =2940a(19) =13860a(20) =14490a(21) =14520a(22) =21840a(23) =31860a(24) =33660a(25) =44580a(26) =80850a(27) =1228080a(28) =4305210a(29) =5326860