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%I #30 Dec 21 2016 11:01:48
%S 9,21,93,381,24573,393213,1572861,6442450941,6917529027641081853,
%T 1856910058928070412348686333,486777830487640090174734030864381,
%U 510423550381407695195061911147652317181
%N 3 times Mersenne primes A000668.
%C Also sum of n-th Mersenne prime and the radical of n-th even perfect number.
%C The binary representation of a(n) has only two zeros, starting with "10" and ending with "01". The sequence begins: 1001, 10101, 1011101, 101111101, 101111111111101,...
%F a(n) = 3*A000668(n) = A000668(n) + A139257(n).
%F a(n) = phi(M(n)) + sigma(sigma(M(n))) = A000010(A000668(n)) + A000203(A000203(A000668(n))) = A000010(A000668(n)) + A051027(A000668(n)).
%Y Cf. A000010, A000203, A000396, A000668, A051027, A147757.
%Y Subsequence of A001748, and of A147758, and of A174055, and possibly of other sequences, see below:
%Y Cf. A060482, A068156, A072087, A136252, A144482, A144856, A249780, A269633.
%K nonn
%O 1,1
%A _Omar E. Pol_, Dec 20 2016