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Divide by 2, multiply by 4, repeat.
4

%I #25 Feb 02 2024 11:02:48

%S 10,5,20,10,40,20,80,40,160,80,320,160,640,320,1280,640,2560,1280,

%T 5120,2560,10240,5120,20480,10240,40960,20480,81920,40960,163840,

%U 81920,327680,163840,655360,327680,1310720,655360,2621440,1310720,5242880

%N Divide by 2, multiply by 4, repeat.

%C A147675-A147678 are from a quiz that someone asked me to help them with.

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,2).

%F From _R. J. Mathar_, Apr 22 2009: (Start)

%F a(n) = 2*a(n-2) = 5*A135530(n-1).

%F G.f.: 5*x*(2+x)/(1-2*x^2). (End)

%F From _Amiram Eldar_, Feb 02 2024: (Start)

%F Sum_{n>=1} 1/a(n) = 3/5.

%F Sum_{n>=1} (-1)^n/a(n) = 1/5. (End)

%t LinearRecurrence[{0, 2}, {10, 5}, 50] (* _Paolo Xausa_, Jan 30 2024 *)

%Y Cf. A020714, A135530.

%K nonn,easy

%O 1,1

%A _N. J. A. Sloane_, Apr 21 2009

%E More terms from _R. J. Mathar_, Apr 22 2009