Numbers k such that k and k-1 are composite.

Terms

a(0) =9a(1) =10a(2) =15a(3) =16a(4) =21a(5) =22a(6) =25a(7) =26a(8) =27a(9) =28a(10) =33a(11) =34a(12) =35a(13) =36a(14) =39a(15) =40a(16) =45a(17) =46a(18) =49a(19) =50a(20) =51a(21) =52a(22) =55a(23) =56a(24) =57a(25) =58a(26) =63a(27) =64a(28) =65a(29) =66