A144856 Semiprimes that are a product of distinct Mersenne primes.

21, 93, 217, 381, 889, 3937, 24573, 57337, 253921, 393213, 917497, 1040257, 1572861, 3670009, 4063201, 16252897, 16646017, 66584449, 1073602561, 4294434817, 6442450941, 15032385529, 66571993057, 68718821377, 272730423169

Offset

1,1

Comments

Since each Mersenne prime is congruent to -1 (mod 4), it is easy to see that a(n) == 1 (mod 4). - Timothy L. Tiffin, Jul 07 2021

Links

T. D. Noe, Table of n, a(n) for n = 1..90

G. L. Honaker, Jr. and Chris Caldwell, Prime Curios! 21

Googology Wiki,Largest known squarefree semiprime

Example

a(1) = 3*7 = 21, a(2) = 3*31 = 93, a(3) = 7*31 = 217, ... - Timothy L. Tiffin, Jul 07 2021

Mathematica

Mp = 2^{2, 3, 5, 7, 13, 17, 19, 31, 61} - 1; Take[ Union[ Times @@@ Subsets[ Mp, {2}]], 25] (* Robert G. Wilson v, Sep 25 2008 *)

Crossrefs

Cf. A000668, A001358. Subsequence of A144482 (semiprimes that are the product of Mersenne primes).

Sequence in context: A143843 A119109 A254603 * A065522 A218844 A219423

Adjacent sequences: A144853 A144854 A144855 * A144857 A144858 A144859

Keyword

nonn

Author

G. L. Honaker, Jr., Sep 22 2008

Extensions

More terms from Robert G. Wilson v, Sep 25 2008

More terms from T. D. Noe, Sep 25 2008

Status

approved