A255452(2^n-1).

Terms

a(0) =1a(1) =5a(2) =21a(3) =77a(4) =277a(5) =1005a(6) =3669a(7) =13421a(8) =49109a(9) =179693a(10) =657493a(11) =2405741a(12) =8802517a(13) =32208109a(14) =117848405a(15) =431203437a(16) =1577759189a(17) =5772968941a(18) =21123103317a(19) =77288739693a(20) =282796954325