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 A176652 Numbers k such that both semiprime(k)/p and semiprime(k+1)/p are prime for some prime p. 1
 1, 2, 4, 6, 21, 42, 87, 120, 141, 142, 168, 179, 185, 188, 245, 255, 320, 363, 387, 434, 464, 496, 539, 593, 675, 697, 721, 753, 794, 810, 894, 929, 995, 1023, 1032, 1060, 1080, 1081, 1105, 1147, 1166, 1221, 1224, 1228, 1275, 1356, 1391, 1477, 1478, 1498 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Indices n such that A001358(n) and A001358(n+1) share one prime factor. - R. J. Mathar, Apr 26 2010 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE 2 is a term because both semiprime(2)/3 = 6/3 = 2 and semiprime(2+1)/3 = 9/3 = 3 are prime. MAPLE isA176652 := proc(n) pfsn := convert(numtheory[factorset]( A001358(n) ), list) ; pfsn1 := convert(numtheory[factorset]( A001358(n+1) ), list) ; op(1, pfsn) = op(1, pfsn1) or op(1, pfsn) = op(-1, pfsn1) or op(-1, pfsn) = op(1, pfsn1) or op(-1, pfsn) = op(-1, pfsn1) ; end proc: for n from 1 to 1600 do if isA176652(n) then printf("%d, ", n) ; end if; end do: # R. J. Mathar, Apr 26 2010 MATHEMATICA sppQ[{a_, b_}]:=Module[{af=FactorInteger[a][[All, 1]], bf=FactorInteger[b][[All, 1]]}, Length[Intersection[af, bf]]==1]; Position[Partition[ Select[ Range[7000], PrimeOmega[#]==2&], 2, 1], _?sppQ]//Flatten (* Harvey P. Dale, Oct 08 2017 *) CROSSREFS Cf. A001358. Sequence in context: A193774 A241210 A333992 * A251724 A326363 A273522 Adjacent sequences: A176649 A176650 A176651 * A176653 A176654 A176655 KEYWORD nonn AUTHOR Juri-Stepan Gerasimov, Apr 22 2010 EXTENSIONS Extended beyond 141 by R. J. Mathar, Apr 26 2010 Name clarified by Jon E. Schoenfield, Feb 06 2019 STATUS approved

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Last modified September 29 02:45 EDT 2023. Contains 365749 sequences. (Running on oeis4.)