Number of subsets of {1,...,n} containing two elements whose difference is 3.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =4a(5) =14a(6) =37a(7) =83a(8) =181a(9) =387a(10) =824a(11) =1728a(12) =3584a(13) =7360a(14) =15032a(15) =30571a(16) =61987a(17) =125339a(18) =252883a(19) =509294a(20) =1024300a(21) =2057848a(22) =4130724a(23) =8285758a(24) =16610841a(25) =33285207a(26) =66673209a(27) =133512759a(28) =267294832a(29) =535025408