Start with n, apply k->2k+1 until a semiprime is reached; sequence gives the semiprimes.

Terms

a(0) =15a(1) =95a(2) =15a(3) =9a(4) =95a(5) =55a(6) =15a(7) =35a(8) =39a(9) =21a(10) =95a(11) =25a(12) =55a(13) =119a(14) =511a(15) =33a(16) =35a(17) =303a(18) =39a(19) =335a(20) =87a(21) =91a(22) =95a(23) =49a(24) =51a(25) =215a(26) =55a(27) =57a(28) =119a(29) =123