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%I A155721 #5 Aug 30 2019 21:51:58
%S A155721 4,20,84,340,1364,5460,21844,87380,349524,1398100,5592404,22369620,
%T A155721 89478484,357913940,1431655764,5726623060,22906492244,91625968980,
%U A155721 366503875924,1466015503700,5864062014804,23456248059220,93824992236884
%N A155721 Positions of parity change in A033035.
%C A155721 Numbers n such that A033035(n)=(7/8)*(9^n - 1)=7*A002452(n).
%F A155721 a(n=1,2..)=(4/3)*(4^n-1)=A080674(n).
%F A155721 a(n) = 5*a(n-1) - 4*a(n-2) for n > 2. G.f.: 4*x/((x - 1)*(4*x - 1)). - _Chai Wah Wu_, Aug 30 2019
%e A155721 A033035(n=1..4) all are odd, then A033035(n=5..20) all are even, then A033035(n=21..84) all are odd, then A033035(n=85..340) all are even, etc.
%Y A155721 A002452, A033035, A080674.
%K A155721 nonn
%O A155721 1,1
%A A155721 _Zak Seidov_, Jan 25 2009

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