Number of subsets of {1,...,n} containing two elements whose difference is 2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =7a(5) =17a(6) =39a(7) =88a(8) =192a(9) =408a(10) =855a(11) =1775a(12) =3655a(13) =7478a(14) =15228a(15) =30898a(16) =62511a(17) =126177a(18) =254223a(19) =511472a(20) =1027840a(21) =2063600a(22) =4140015a(23) =8300767a(24) =16635087a(25) =33324462a(26) =66736764a(27) =133615658a(28) =267461287a(29) =535294673