A101618 Indices of semiprimes in A001003.

7, 11, 17, 19, 21, 239

Offset

1,1

Comments

No more terms < 2000. - David Wasserman, Mar 27 2008

a(7) >= 4377. A001003(4377) is a 3345-digit composite number with unknown factorization. Other possible terms are 4391, 45207, 45369, 45377, 131723, 131733, ... - Tyler Busby, Feb 08 2023

Links

Table of n, a(n) for n=1..6.

Eric Weisstein's World of Mathematics, Semiprime.

Formula

A001003(a(n)) is in A001358 (semiprimes).

Example

a(1) = 7 because A001003(7) = 4279 = 11 * 389.
a(2) = 11 because A001003(11) = 2646723 = 3 * 882241.
a(3) = 17 because A001003(17) = 55909013009 = 41263 * 1354943.
a(4) = 19 because A001003(19) = 1618362158587 = 24413 * 66290999.
a(5) = 21 because A001003(21) = 47574827600981 = 3253 * 14624908577.

Crossrefs

Cf. A001003, A001358.

Cf. A101619, A092839, A092840.

Sequence in context: A296305 A349997 A076045 * A147956 A163996 A117743

Adjacent sequences: A101615 A101616 A101617 * A101619 A101620 A101621

Keyword

nonn,more,less

Author

Jonathan Vos Post, Dec 09 2004

Extensions

Edited by Franklin T. Adams-Watters, Nov 07 2006

Changed by Michael Somos, Mar 31 2007, because of the re-indexing of A001003.

a(6) from David Wasserman, Mar 27 2008

Status

approved