A binomial coefficient summation.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =6a(5) =19a(6) =49a(7) =163a(8) =472a(9) =1626a(10) =5034a(11) =17769a(12) =57474a(13) =206487a(14) =688881a(15) =2508195a(16) =8563020a(17) =31504240a(18) =109492960a(19) =406214878a(20) =1432030036a(21) =5349255726a(22) =19077934506a(23) =71672186953a(24) =258095737156a(25) =974311431094