Partial sums of A002861.

Terms

a(0) =1a(1) =3a(2) =7a(3) =16a(4) =36a(5) =87a(6) =212a(7) =541a(8) =1403a(9) =3714a(10) =9931a(11) =26880a(12) =73230a(13) =200944a(14) =554216a(15) =1535969a(16) =4273508a(17) =11933297a(18) =33425583a(19) =93891713a(20) =264401743a(21) =746269426a(22) =2110694255a(23) =5981068081a(24) =16977958318a(25) =48271041858