Number of subsets S of {1,2,3,...,n} with the property that if x is a member of S then at least one of x-2 and x+2 is also a member of S.
A172020
Number of subsets S of {1,2,3,...,n} with the property that if x is a member of S then at least one of x-2 and x+2 is also a member of S.
Terms
- a(0) =1a(1) =1a(2) =2a(3) =4a(4) =8a(5) =16a(6) =28a(7) =49a(8) =84a(9) =144a(10) =252a(11) =441a(12) =777a(13) =1369a(14) =2405a(15) =4225a(16) =7410a(17) =12996a(18) =22800a(19) =40000a(20) =70200a(21) =123201a(22) =216216a(23) =379456a(24) =665896a(25) =1168561a(26) =2050657a(27) =3598609a(28) =6315113a(29) =11082241
External references
- oeis: A172020