Walks of length n on the x-axis using steps {1,-1} and visiting no point more than twice.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =12a(5) =18a(6) =26a(7) =40a(8) =56a(9) =82a(10) =112a(11) =162a(12) =222a(13) =316a(14) =432a(15) =608a(16) =834a(17) =1166a(18) =1602a(19) =2228a(20) =3066a(21) =4252a(22) =5858a(23) =8108a(24) =11178a(25) =15454a(26) =21316a(27) =29450a(28) =40632a(29) =56114