Occurrences of principal character.

Terms

a(0) =1a(1) =0a(2) =1a(3) =3a(4) =6a(5) =15a(6) =42a(7) =109a(8) =320a(9) =979a(10) =3137a(11) =10603a(12) =37728a(13) =140423a(14) =546859a(15) =2220294a(16) =9374309a(17) =41059926a(18) =186149869a(19) =871717073a(20) =4208841555a(21) =20917405824a(22) =106849492793