The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A111171 Semiprimes S such that 3*S - 1 is also a semiprime. 6
 9, 21, 22, 25, 26, 49, 62, 65, 69, 74, 85, 93, 121, 122, 129, 133, 141, 146, 158, 161, 166, 178, 185, 194, 205, 209, 221, 249, 253, 262, 265, 289, 298, 302, 305, 309, 346, 358, 361, 365, 381, 382, 386, 413, 446, 466, 473, 485, 489, 493, 501, 505, 514, 526, 553 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is analogous to Sophie Germain semiprimes A111153 and the chains shown are analogous to Cunningham chains of the second kind and Tomaszewski chains of the second kind. Define a 3n-1 semiprime chain of length k. This is a sequence of semiprimes s(1) < s(2) < ... < s(k) such that s(i+1) = 3*s(i) - 1 for i = 1, ..., k-1. Length 3: 9, 26, 77; 49, 146, 437; 65, 194, 581; 129, 386, 1157; 158, 473, 1418; 187, 562, 1685. Length 4: 74, 221, 662, 1985; 122, 365, 1094, 3281. Length 5: 21, 62, 185, 554, 1661. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 FORMULA {a(n)} = a(n) is an element of A001358 and 3*a(n)-1 is an element of A001358. EXAMPLE n s(n) 3 *s -1 1 9 = 3^2 26 = 2 * 13 2 21 = 3 * 7 62 = 2 * 31 3 22 = 2 * 11 65 = 5 * 13 4 25 = 5^2 74 = 2 * 37 5 26 = 2 * 13 77 = 7 * 11 6 49 = 7^2 146 = 2 * 73 MATHEMATICA Select[Range[600], PrimeOmega[#]==PrimeOmega[3#-1]==2&] (* Harvey P. Dale, Jun 20 2018 *) CROSSREFS Cf. A001358, A111153, A111168, A111170, A111173, A111176. Sequence in context: A259250 A251219 A284131 * A317789 A333039 A266000 Adjacent sequences:  A111168 A111169 A111170 * A111172 A111173 A111174 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Oct 21 2005 EXTENSIONS Corrected and extended by Ray Chandler, Oct 22 2005 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 25 20:01 EDT 2021. Contains 347659 sequences. (Running on oeis4.)