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

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A233562 Products p*q of distinct primes such that (p*q + 1)/2 is a prime. 4
 21, 33, 57, 85, 93, 133, 141, 145, 177, 201, 205, 213, 217, 253, 301, 381, 393, 445, 453, 481, 501, 537, 553, 565, 633, 697, 717, 745, 793, 817, 865, 913, 921, 933, 973, 1041, 1081, 1137, 1141, 1261, 1285, 1293, 1317, 1345, 1401, 1417, 1437, 1465, 1477, 1501 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence is a subsequence of A128283 since the condition that (p+q)/2 be prime is not required here. The smallest number not in A128283 is 141=3*47 since (3+47)/2=25. - Hartmut F. W. Hoft, Oct 31 2020 LINKS EXAMPLE 21 = 3*7 is the least product of distinct primes p and q for which (p*q + 1)/2 is a prime, so a(1) = 21. MATHEMATICA t = Select[Range[1, 7000, 2], Map[Last, FactorInteger[#]] == Table[1, {2}] &]; Take[(t + 1)/2, 120] (* A234096 *) v = Flatten[Position[PrimeQ[(t + 1)/2], True]] ; w = Table[t[[v[[n]]]], {n, 1, Length[v]}]  (* A233562 *) (w + 1)/2 (* A234098 *)    (* Peter J. C. Moses, Dec 23 2013 *) With[{nn=50}, Take[Union[Select[Times@@@Subsets[Prime[Range[2nn]], {2}], PrimeQ[ (#+1)/2]&]], nn]] (* Harvey P. Dale, Mar 24 2015 *) CROSSREFS Cf. A233561, A046388. Cf. A128283. Sequence in context: A271101 A191683 A032603 * A128283 A280878 A033901 Adjacent sequences:  A233559 A233560 A233561 * A233563 A233564 A233565 KEYWORD nonn,easy AUTHOR Clark Kimberling, Dec 14 2013 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 November 23 14:23 EST 2020. Contains 338590 sequences. (Running on oeis4.)