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a(n) = phi(2*prime(n)+1).
1

%I #17 Jun 20 2023 06:33:27

%S 4,6,10,8,22,18,24,24,46,58,36,40,82,56,72,106,96,80,72,120,84,104,

%T 166,178,96,168,132,168,144,226,128,262,200,180,264,200,144,216,264,

%U 346,358,220,382,252,312,216,276,296,288,288,466,478,264,502,408,480,420,360,288,562

%N a(n) = phi(2*prime(n)+1).

%C 2*prime(n)+1 is prime iff a(n) = 2*prime(n).

%F a(n) = A000010(A072055(n)).

%F a(n) = A037225(A000040(n)).

%t a[n_] := EulerPhi[2*Prime[n] + 1]; Array[a, 100] (* _Amiram Eldar_, Jun 16 2023 *)

%o (PARI) a(n)=eulerphi(2*prime(n)+1)

%Y Cf. A000010, A000040, A008331, A037225, A072055.

%K nonn

%O 1,1

%A _Alain Rocchelli_, Jun 16 2023