A101757 Indices of semiprimes in A000073.

5, 12, 18, 42, 44, 50, 58, 67, 74, 83, 90, 95, 126, 142, 194, 210, 211, 275, 367, 382, 479, 655, 734, 818, 839, 926, 939, 1047

Offset: 1

Author

Jonathan Vos Post and Ray Chandler, Dec 26 2004

Keywords

• nonn
• more
• less

Comments

a(29) >= 1091. A000073(1091) is a 288-digit composite number with unknown factorization. Other possible terms are 1390, 1630, 2034, 2131, 2339, ... - Tyler Busby, Feb 09 2023

Links

• Eric Weisstein's World of Mathematics, Semiprime

Crossrefs

Cf. A000073, A001358, A101758.

Equivalent sequence for primes: A303263.

Programs

Mathematica

Flatten[Position[LinearRecurrence[{1,1,1},{1,1,2},100],?(PrimeOmega[ #]==2&)]]+1 (* This generates the first 12 terms of the sequence.  To generate more, increase 100 to a higher number, but it will take increasingly long to run. *) (* _Harvey P. Dale, May 31 2013 *)

Formula

A000073(a(n)) = A101758(n).

Extensions

More terms from David Wasserman, Apr 03 2008