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%I #10 Sep 08 2022 08:45:48
%S 1,7,3,21,9,63,27,189,81,567,243,1701,729,5103,2187,15309,6561,45927,
%T 19683,137781,59049,413343,177147,1240029,531441,3720087,1594323,
%U 11160261,4782969,33480783,14348907,100442349,43046721,301327047
%N a(n) = 3*a(n-2) for n > 2; a(1) = 1; a(2) = 7.
%C Interleaving of A000244 and A005032.
%C Seventh binomial transform is A153598.
%H Vincenzo Librandi, <a href="/A166481/b166481.txt">Table of n, a(n) for n = 1..200</a>
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,3).
%F a(n) = (5+2*(-1)^n)*3^(1/4*(2*n-5+(-1)^n)).
%F G.f.: x*(1+7*x)/(1-3*x^2).
%t LinearRecurrence[{0,3},{1,7},50] (* or *) Flatten[NestList[3#&,{1,7},20]] (* _Harvey P. Dale_, Sep 24 2015 *)
%o (Magma) [ n le 2 select 6*n-5 else 3*Self(n-2): n in [1..34] ];
%Y Cf. A000244 (powers of 3), A005032 (7*3^n), A153598.
%K nonn
%O 1,2
%A _Klaus Brockhaus_, Oct 14 2009
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