This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A279389 #30 Dec 21 2016 11:01:48 %S A279389 9,21,93,381,24573,393213,1572861,6442450941,6917529027641081853, %T A279389 1856910058928070412348686333,486777830487640090174734030864381, %U A279389 510423550381407695195061911147652317181 %N A279389 3 times Mersenne primes A000668. %C A279389 Also sum of n-th Mersenne prime and the radical of n-th even perfect number. %C A279389 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 A279389 a(n) = 3*A000668(n) = A000668(n) + A139257(n). %F A279389 a(n) = phi(M(n)) + sigma(sigma(M(n))) = A000010(A000668(n)) + A000203(A000203(A000668(n))) = A000010(A000668(n)) + A051027(A000668(n)). %Y A279389 Cf. A000010, A000203, A000396, A000668, A051027, A147757. %Y A279389 Subsequence of A001748, and of A147758, and of A174055, and possibly of other sequences, see below: %Y A279389 Cf. A060482, A068156, A072087, A136252, A144482, A144856, A249780, A269633. %K A279389 nonn %O A279389 1,1 %A A279389 _Omar E. Pol_, Dec 20 2016