A279389 3 times Mersenne primes A000668.

9, 21, 93, 381, 24573, 393213, 1572861, 6442450941, 6917529027641081853, 1856910058928070412348686333, 486777830487640090174734030864381, 510423550381407695195061911147652317181

Offset: 1

Author

Omar E. Pol, Dec 20 2016

Keywords

• nonn

Comments

Also sum of n-th Mersenne prime and the radical of n-th even perfect number.

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,...

Crossrefs

Cf. A000010, A000203, A000396, A000668, A051027, A147757.

Subsequence of A001748, and of A147758, and of A174055, and possibly of other sequences, see below:

Cf. A060482, A068156, A072087, A136252, A144482, A144856, A249780, A269633.

Formula

a(n) = 3*A000668(n) = A000668(n) + A139257(n).

a(n) = phi(M(n)) + sigma(sigma(M(n))) = A000010(A000668(n)) + A000203(A000203(A000668(n))) = A000010(A000668(n)) + A051027(A000668(n)).