[ exp(2/7)*n! ].

Terms

a(0) =1a(1) =2a(2) =7a(3) =31a(4) =159a(5) =958a(6) =6706a(7) =53654a(8) =482888a(9) =4828888a(10) =53117772a(11) =637413271a(12) =8286372532a(13) =116009215452