Number of doubly-surjective functions f:[n]->[3].

Terms

a(0) =90a(1) =630a(2) =2940a(3) =11508a(4) =40950a(5) =137610a(6) =445896a(7) =1410552a(8) =4390386a(9) =13514046a(10) =41278068a(11) =125405532a(12) =379557198a(13) =1145747538a(14) =3452182656a(15) =10388002848a(16) =31230066186a(17) =93828607686a(18) =281775226860a(19) =845929656900