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 A216957 a(1)=2; for n > 1, a(n) = 2^(n-2) + (1/(2n-2)) * Sum_{ d divides n-1 } phi(2d)*2^((n-1)/d). 2

%I

%S 2,2,4,6,12,20,40,74,148,286,568,1118,2228,4412,8788,17480,34836,

%T 69392,138388,275942,550560,1098516,2192572,4376666,8738324,17448308,

%U 34845304,69594398,139011816,277691852,554767744,1108378658,2214594580,4425117884,8842583584,17670722600,35314182976,70576759892,141055781836

%N a(1)=2; for n > 1, a(n) = 2^(n-2) + (1/(2n-2)) * Sum_{ d divides n-1 } phi(2d)*2^((n-1)/d).

%H N. J. A. Sloane, <a href="/A216957/b216957.txt">Table of n, a(n) for n = 1..1000</a>

%p with(numtheory);

%p f:=n-> if n=1 then 2 else 2^(n-2) + (1/(2*n-2)) * add(phi(2*d)*2^((n-1)/d), d in divisors(n-1)); fi;

%Y Different from, but easily confused with, A003000 and A122536.

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Sep 26 2012

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Last modified September 25 08:41 EDT 2020. Contains 337335 sequences. (Running on oeis4.)