Nearest integer to exponential integral of n.

Terms

a(0) =2a(1) =5a(2) =10a(3) =20a(4) =40a(5) =86a(6) =192a(7) =440a(8) =1038a(9) =2492a(10) =6071a(11) =14960a(12) =37198a(13) =93193a(14) =234956a(15) =595561a(16) =1516638a(17) =3877904a(18) =9950907a(19) =25615653a(20) =66127186a(21) =171144671a(22) =443966370a(23) =1154115392a(24) =3005950907a(25) =7842940992a(26) =20496497120a(27) =53645118592a(28) =140599195758