Number of happy numbers <= 10^n.

Terms

a(0) =1a(1) =3a(2) =20a(3) =143a(4) =1442a(5) =14377a(6) =143071a(7) =1418854a(8) =14255667a(9) =145674808a(10) =1492609148a(11) =15091199357a(12) =149121303586