a(n) = round(Chi(n)) where Chi(x) is the cosh integral function.

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =20a(5) =43a(6) =96a(7) =220a(8) =519a(9) =1246a(10) =3036a(11) =7480a(12) =18599a(13) =46596a(14) =117478a(15) =297780a(16) =758319a(17) =1938952a(18) =4975454a(19) =12807826a(20) =33063593a(21) =85572336a(22) =221983185a(23) =577057696a(24) =1502975453a(25) =3921470496a(26) =10248248560a(27) =26822559296a(28) =70299597879a(29) =184486604704