Number of subsets of {1,.., n} containing exactly two primes.

Terms

a(0) =0a(1) =0a(2) =2a(3) =4a(4) =12a(5) =24a(6) =48a(7) =96a(8) =192a(9) =384a(10) =640a(11) =1280a(12) =1920a(13) =3840a(14) =7680a(15) =15360a(16) =21504a(17) =43008a(18) =57344a(19) =114688a(20) =229376a(21) =458752a(22) =589824a(23) =1179648a(24) =2359296a(25) =4718592a(26) =9437184a(27) =18874368a(28) =23592960a(29) =47185920