Number of subsets A of {1..n} such that there are no solutions to a+b+c=d for a,b,c,d in A.

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =11a(5) =21a(6) =31a(7) =55a(8) =99a(9) =145a(10) =252a(11) =430a(12) =620a(13) =1042a(14) =1786a(15) =2597a(16) =4304a(17) =7241a(18) =10374a(19) =17098a(20) =28967a(21) =41444a(22) =68017a(23) =113746a(24) =162204a(25) =268412a(26) =449318a(27) =640341a(28) =1053604a(29) =1764648