a(n) is the index of the first occurrence of n in A371091.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =21a(5) =51a(6) =111a(7) =321a(8) =741a(9) =2001a(10) =4311a(11) =8931a(12) =22791a(13) =52821a(14) =112881a(15) =293061a(16) =803571a(17) =1824591a(18) =4887651a(19) =14587341a(20) =33986721a(21) =92184861a(22) =208581141a(23) =431674011a(24) =877859751a(25) =2216416971a(26) =4893531411a(27) =11363224641a(28) =24302611101a(29) =63120770481